Light Energy

Light is energy enabling sight, traveling in straight rays. Sources are natural (Sun, stars) and artificial (bulbs, candles).

Key properties include rectilinear propagation (straight-line travel, causing shadows) and reflection (bouncing off surfaces).

Reflection follows laws: incident ray, reflected ray, and normal are coplanar; angle of incidence equals angle of reflection.

Types of reflection are regular (smooth surfaces, clear images) and irregular/diffused (rough surfaces, scattered light, seeing non-shiny objects).

Shadows form when opaque objects block light’s straight path, their shape depending on the object and light source.

Test Yourself
A. Objective Questions 

1. Write true or false for each statement

(a) The image formed by a plane mirror is real.
Ans: False.
The image formed by a plane mirror is virtual.

(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the angle of reflection.
Ans: False.
When a light ray is reflected from a wall, the angle of incidence is equal to the angle of reflection.

(c) The image of the right hand in a plane mirror looks like that of a left hand.
Ans: True.

(d) The image formed by a plane mirror is upright.
Ans: True.

(e) The image formed by a plane mirror can be obtained on a screen.
Ans: False.
The image formed by a plane mirror cannot be obtained on a screen.

(f) The objects are seen around us due to irregular reflection of light.
Ans: True.

(g) The speed of light in vacuum is 3 × 108 ms-1.
Ans: True.

(h) A rose appears red in light of all the colours.
Ans: False. A rose appears red in white light.

(i) A black paper absorbs light of all the colours and reflects none.
Ans: True.

(j) The primary colours are red, blue and green.
Ans: True.

2. Fill in the blanks

(a) Angle of incidence = ——–

Ans : angle of reflection
(b) The incident ray, the reflected ray and the normal lie in one ——–

Ans : plane
(c) The image formed by a plane mirror is at a distance behind the mirror as ———.

Ans : the object is in front of it
(d) The image formed by a plane mirror is erect and ———.

Ans : virtual
(e) We are able to see the objects around us due to ——- reflection.

Ans : irregular
(f) A ——– image cannot be obtained on a screen.

Ans : virtual
(g) One surface of mirror is made opaque by ———– it followed by a thin coating of paint of lead oxide.

Ans : silvering
(h) A plane mirror ——— reflect 100 percent light falling on it.

Ans : does not
(i) The colour of an opaque object is the colour of light which it ——.

Ans : reflects
(j) Magenta, cyan and yellow are the ——– colours.

Ans : secondary

3. Match the following

4. Select the correct alternative

(a) A man standing in front of a plane mirror finds his image to be at a distance of 6 metre from himself. The distance of man from the mirror is

1. 6 m
2. 3 m
3. 2 m
4. 12 m

(b) The angle between the incident ray and the ray reflected from the plane mirror is 70°. The angle of incidence will be :

1. 70°
2. 30°
3. 35°
4. 90°

(c) The image formed by a plane mirror is

1. virtual and inverted
2. virtual and of same size
3. real and inverted
4. real and of same size

(d) The angle of incidence on a plane mirror is 30°.The angle of reflection will be:

1. 30°
2. 60°
3. 15°
4. 0°

(e) The angle of incidence on a plane mirror is 30°. The angle between the incident ray and the reflected ray is

1. 30°
2. 15°
3. 60°
4. 90°

(f) The property due to which a light ray striking a surface is returned back into the same medium is called

1. refraction
2. reflex action
3. reflection
4. regression

(g) A ray of light after reflection from a mirror is known as

1. reflected ray
2. normal
3. incident ray
4. refracted ray

(h) The speed of light is maximum in

1. glass
2. water
3. air
4. wood

(i) A red rose is seen in green light. It will appear.

1. red
2. blue
3. yellow
4. black

(j) The primary colours are

1. Blue, Green and Red
2. Red, Blue and Yellow
3. Magenta, Yellow and Cyan
4. Red, Blue and Cyan

B. Short/Long Answer Questions

Question 1.
What do you mean by the term reflection of light ?
Ans:

Reflection of light is when light rays bounce back or change direction after hitting a surface.

Question 2.

How is a plane mirror made ?

Ans:

Plane Mirror Creation :

A plane mirror is made by applying a thin layer of reflective metal (usually silver or aluminum) to the back of a flat, smooth piece of glass.

Process:

1. Clean Glass: The glass is meticulously cleaned.
2. Reflective Coating: Silver or aluminum is deposited on the back, often via vacuum deposition for uniformity.
3. Protective Layer: A paint or special coating is applied over the metal to prevent scratches and corrosion, ensuring the mirror’s longevity and reflectivity.

Question 3.

Explain the following terms:

Incident ray, Reflected ray, Angle of incidence, Angle of reflection, Normal.

Ans:

• Incident Ray: The path taken by a light particle as it travels towards a reflective surface.
• Reflected Ray: The path taken by the same light particle after it bounces off the reflective surface at the point of incidence.
• Angle of Incidence (∠i): The precise angle formed between the incoming incident ray and an imaginary line, known as the normal, that is drawn perpendicularly to the reflecting surface at the exact point where the light strikes.
• Angle of Reflection (∠r): The precise angle formed between the outgoing reflected ray and the same imaginary line, the normal, at the point of incidence where the light was redirected.
• Normal: An entirely conceptual line, constructed to be exactly 90 degrees to the plane of the reflecting surface at the specific location where the incident light ray makes contact. It serves as the crucial reference line for measuring the angles of incidence and reflection.

Question 4.

Draw a diagram showing the reflection of a light ray from a plane mirror. Label on it the incident ray, the reflected ray, the normal, the angle of incidence i and the angle of reflection r.

Ans:

Envision a level, linear representation of a mirror’s surface oriented horizontally. Projecting perpendicularly from a specific point along this horizontal line is a vertical line, designated as the normal. An incoming ray of light, depicted as a directional arrow, travels towards and makes contact with the mirror precisely at the base of this normal line, thereby becoming the incident ray. Subsequently, another directional arrow originates from this same point of contact, illustrating the light ray’s path as it rebounds from the mirror’s surface, thus representing the reflected ray. The angle formed between the incident ray and the normal is labeled as ‘i’, signifying the angle of incidence. Similarly, the angle created between the reflected ray and the normal is labeled as ‘r’, indicating the angle of reflection.

Question 5.

State the two laws of reflection of light.

Ans:

The fundamental principles governing the behavior of light upon reflection are encapsulated in two distinct laws:

1. Law of Coplanarity: The incoming ray of light (the incident ray), the outgoing ray of light after striking the surface (the reflected ray), and the imaginary line drawn perpendicularly to the reflecting surface at the exact point where the incident ray makes contact (the normal) invariably reside within a single, common plane.
   
2. Law of Equal Angles: The precise angle formed between the incident ray and the normal (the angle of incidence, denoted as ∠i) is exactly equivalent in measure to the precise angle formed between the reflected ray and the normal (the angle of reflection, denoted as ∠r).

Question 6.

Describe an experiment to verify the laws of reflection of light. Ans. Laws of reflection.

Ans:

Experiment to Verify Reflection Laws :

1. Setup: Fix paper on a board, stand a mirror.
2. Incident Ray: Draw a line (AO), place two pins (P, Q) along it.
3. Reflected Ray: View pin images, place two more pins (R, S) in line with them.
4. Trace: Remove pins, draw reflected ray (OB) connecting R, S to where AO hit the mirror (O).
5. Normal: Draw a line (ON) perpendicular to the mirror at O.
6. Measure: Use a protractor to find angle of incidence (∠AON = i) and angle of reflection (∠BON = r).

Verification:

• Incident ray (AO), reflected ray (OB), and normal (ON) will be on the paper (same plane – 1st law).
• Measured angles ‘i’ and ‘r’ will be approximately equal (2nd law).

Repeat with different incident ray angles to confirm.

Question 7.

A ray of light falls normally on a plane mirror. What is the angle of incidence ?

Ans:

When a light ray strikes a plane mirror in a manner that is exactly perpendicular to the mirror’s surface, this is termed normal incidence. The angle of incidence is specifically defined as the angular separation between the path of the incoming light ray and an imaginary line constructed to be exactly perpendicular to the reflecting surface at the precise point where the light makes contact. In this particular scenario of normal incidence, the incoming light ray is already aligned perfectly with this perpendicular imaginary line (the normal). Consequently, the angular separation between the incident ray and the normal is zero degrees. Therefore, under conditions of normal incidence, the angle of incidence is 0°.

Question 8.

Draw a diagram to show the reflection of a light ray incident normally on a plane mirror.

Ans:

Draw a horizontal line (mirror). Draw a single vertical arrow hitting the mirror straight on and bouncing straight back along the same path. This vertical line is also the normal. Label the incoming arrow “Incident Ray,” the outgoing arrow “Reflected Ray,” and the vertical line “Normal.” Indicate that the angle between the incident ray and the normal (angle of incidence, ‘i’) is 0°, and the angle between the reflected ray and the normal (angle of reflection, ‘r’) is also 0°.

Question 9.

The diagram in Fig. shows an incident ray AO and the reflected ray OB from a plane mirror. The angle AOB is 30°. Draw normal on the plane mirror at the point O and find :

Ans:

Mirror is a horizontal line. Point ‘O’ is where light hits. Incident ray (AO) and reflected ray (OB) form a 30° angle (∠AOB).

Draw Normal: A vertical line straight up from ‘O’ on the mirror.

Find Angles:

Angle of Incidence (∠i): Angle between AO and Normal = 30° / 2 = 15°.

Angle of Reflection (∠r): Angle between OB and Normal = Angle of Incidence = 15°.

Question 10.

In the following diagrams, measure and write the angle of incidence and draw the reflected ray in each case.

Ans:

For each diagram:

1. Draw Normal: Perpendicular line at where light hits the mirror.
2. Measure ‘i’: Angle between incoming light and Normal.
3. Draw Reflected Ray: Bounces off at the same angle as ‘i’ (angle ‘r’), on the other side of the Normal.

Question 11.
The diagram in fig. shows an incident ray AO and the normal ON on a plane mirror. Draw the reflected ray. State the law you use to draw the direction of the reflected ray.

Ans:

Question 12.
The following diagram shows an incident ray AO and the normal ON on a plane mirror. Find the angle of incidence and angle of reflection.

Ans:

Law of reflection of light is used to draw the direction of the reflected ray.
This law states that angle of incidence is equal to the angle of reflection.
∠i = ∠r

Question 13.

State in words, how do you find the location of image of an object formed by a plane mirror.

Ans:

The image in a plane mirror appears to be located as far behind the mirror as the object is in front of it.

Question 14.
Draw a ray diagram showing the formation of image of a point object by a plane mirror.
Ans:

Question 15.
The following diagram shows a point object O placed in front of a plane mirror. Take two rays from the point O and show how the image of O is formed and seen by the eye.

Ans:

Question 16.

State four characteristics of the image formed by a plane mirror.

Ans:

The image formed by a plane mirror possesses four key characteristics:

1. Virtual Nature: The image appears to exist behind the mirror’s surface. It cannot be projected onto a physical screen because the light rays do not actually converge at the image’s location; instead, they diverge as if originating from a point behind the reflecting surface.
   
2. Upright Orientation: The image maintains the same vertical orientation as the object. If the object is positioned upright, its reflection will also appear upright, with no inversion of top and bottom.
   
3. Lateral Inversion: The image exhibits a reversal of left and right. When the object’s left side faces the mirror, the image’s corresponding side will appear on the right, creating a mirror-image effect across a vertical axis.
   
4. Equal Dimensions: The size and proportions of the image are exactly the same as those of the object. A plane mirror does not magnify or diminish the object’s size; the reflection is a true-to-scale representation.

Question 17.

How is the position of image formed by a plane mirror related to the position of the object ?

Ans:

The spatial relationship between an object and its corresponding image formed by a plane mirror is characterized by a precise symmetry:

• Equidistant Placement: The perceived location of the image is situated directly behind the mirror’s reflective surface at a distance that is precisely equivalent to the distance separating the object from the front of the mirror.
  
• Perpendicular Alignment: The image of any specific point on the object is consistently positioned along a straight line that extends perpendicularly from that point, passes through the plane of the mirror, and continues an equal distance on the opposite side.
  
• Lateral Reversal: Although the distance between the object and the mirror is mirrored by the distance between the image and the mirror, the image exhibits a left-to-right reversal relative to the object. This inversion does not alter the symmetrical distance relationship but is a fundamental characteristic of reflections in plane mirrors.
  

In essence, a plane mirror generates a virtual image that is positioned symmetrically opposite the object with respect to the mirror’s surface, maintaining an equal distance behind as the object is in front, and lying along the perpendicular projection of the object through the mirror, albeit with a lateral inversion.

Question 18.

You are standing at a distance 2 metre from a plane mirror.

(a) What is the distance of your image from the mirror ?

(b) What is the distance between you and your image ?

Ans:

(a) Distance of your image from the mirror:

Given that your separation from the plane mirror is 2 metres, the fundamental property of image formation by such a mirror dictates that the perceived location of your reflection will be situated directly behind the mirror at an equivalent distance. Consequently, the distance of your image from the plane mirror is 2 metres.

(b) Distance between you and your image:

To ascertain the total separation between your physical self and your reflected image, we must consider the sum of two distances: your distance from the mirror and the image’s distance from the mirror.

Total distance = (Your distance from the mirror) + (Your image’s distance from the mirror) Total distance = 2 metres + 2 metres = 4 metres.

Ans:

(a) 2 metres (b) 4 metres

Question 19.

What is meant by lateral inversion of an image in a plane mirror ? Explain it with the help of a diagram.

Ans:

Lateral inversion in a plane mirror is the effect where the left side of an object is seen as the right side of its reflection, and conversely, the right side of the object becomes the left side of the image. It’s a side-to-side reversal, like the image has been flipped horizontally. A simple example is when you look in a mirror and raise your right hand; your reflection appears to raise its left hand. The top and bottom orientation, however, remains unchanged.

Question 20.
Wirte down the letter C and I as seen in a plane mirror.
Ans:

Question 21.

What is irregular reflection ? Give an example.

Ans:

Irregular reflection is when light hits a rough surface and gets scattered in many directions, instead of reflecting uniformly. Think of light bouncing off a wooden table or a wall – you can see it from different angles because the light is scattered.

Question 22.

How do we see objects around us ?

Ans:

We see objects because light interacts with them and then enters our eyes. Most objects don’t produce their own light; instead, they reflect light from sources like the sun or lamps. When light strikes an object, certain colors are absorbed, while others are reflected. The color we perceive is the color of light that bounces off the object’s surface. This reflected light travels to our eyes.

Once the reflected light enters our eyes, it passes through the cornea and pupil, and the lens focuses it onto the retina. The retina contains specialized cells that convert this light into electrical signals. These signals are then sent along the optic nerve to our brain. Our brain processes these signals, interpreting them as the images we see, allowing us to perceive the shape, color, and position of the objects around us.

Question 23.

State two uses of a plane mirror.

Ans:

Here are two common uses of a plane mirror:

1. Looking Glass: The most common use of a plane mirror is as a looking glass or dressing mirror. We use it daily to see our reflection for grooming, checking our appearance, and similar purposes. The image formed is upright, the same size as the object, and laterally inverted, allowing us to see ourselves as others see us (with left and right reversed).
   
2. Rearview Mirrors (in some applications): While most modern vehicles use convex mirrors for a wider field of view, plane mirrors are still used in some interior rearview mirrors or in specific applications like buses or trucks for a true-to-size, non-distorted view of what’s directly behind. They provide an accurate representation of distances, although with a narrower field of view compared to convex mirrors.

Question 24.

Can light travel in vacuum ?

Ans:

Absolutely, light possesses the remarkable ability to travel through a vacuum.

In contrast to sound, which necessitates a material substance (such as air, water, or solids) to propagate, light is a form of electromagnetic radiation. These electromagnetic waves are fluctuations in electric and magnetic fields, and crucially, these fields can exist and spread even in regions devoid of matter.

A compelling real-world example is the journey of sunlight. The light emitted by the sun traverses the immense emptiness of space to eventually reach our planet, unequivocally demonstrating light’s capacity to travel in a vacuum. If a medium were required for its transmission, the light from celestial bodies like the sun and stars would never reach us.

Question 25.

State the speed of light in (a) air, (b) glass.

Ans:

(a) The speed of light in air is about 300,000 kilometers per second (3 x 10⁸ m/s). It’s just a tiny bit slower than its speed in the complete emptiness of space.

(b) When light travels through glass, it slows down significantly to roughly 200,000 kilometers per second (2 x 10⁸ m/s). The exact speed depends on the specific type of glass.

Question 26.

State whether light slows down or speeds up in the following cases :

(a) Light going from air to glass.

(b) Light going from glass to water.

(c) Light going from water to air.

Ans:

Here’s how the speed of light changes in each scenario:

(a) Air to glass: Light decreases in speed. Glass is optically denser than air, causing light to propagate more slowly within it.

(b) Glass to water: Light increases in speed. Water is optically less dense than glass, allowing light to travel faster.

(c) Water to air: Light increases in speed. Air is optically less dense than water, enabling light to move more swiftly.

Question 27.

What are the primary colours ? Name the three primary colours.

Ans:

Primary colors are a specific group of colors that are fundamental because they can be mixed together in various proportions to produce a vast array of other colors. A key characteristic of primary colors is that they themselves cannot be created by mixing other colors.

The three colors traditionally recognized as the primary colors are:

• Red
• Yellow
• Blue

Question 28.

What are the secondary colours ? Name the three secondary colours.

Ans:

Secondary colors are those vibrant shades that emerge when you blend two primary colors together. The magic happens when these fundamental colors unite in roughly equal measures, giving birth to entirely new hues.

The familiar trio of secondary colors includes:

• Orange: A lively fusion of the warmth of red and the brightness of yellow.
• Green: The refreshing result of combining the coolness of blue with the cheerfulness of yellow.
• Purple (or Violet): A rich and intriguing mix created by the boldness of red and the serenity of blue.

Question 29.

Fill in the blanks with the appropriate colour

(a) Blue + ………… = Cyan

(b) Red + Blue + …………. = Vhite

(c) Red + Blue = …………

(d) Green + Red = …………

Ans:

Here are the answers to your fill-in-the-blanks:

(a) Blue + Green = Cyan 

(b) Red + Blue + Green = White

 (c) Red + Blue = Magenta 

(d) Green + Red = Yellow

Question 30.

The leaves appear green when seen in white light. Give a reason.

Ans:

The vibrant green hue we associate with leaves stems from their unique way of handling white light. Within their cells resides chlorophyll, a vital pigment for photosynthesis. This remarkable substance acts like a light sponge, eagerly soaking up most colors within the visible spectrum, with a particular fondness for the red and blue wavelengths – the energy powerhouses for the plant’s food-making process. However, green light is a different story. Chlorophyll doesn’t absorb it effectively. As a result, when sunlight or white light bathes a leaf, the red and blue components are mostly captured, while the green wavelengths are largely bounced back, reflecting away from the leaf’s surface. It’s this reflected green light that ultimately enters our eyes, and our brains translate this specific light frequency into the color we perceive as green. In essence, leaves appear green because they selectively send back the green part of the light that shines upon them.

Question 31.

A rose appears red in white light. How will it appear in

(i) green light, (ii) red light ? Give a reason for your answer for each.

Ans:

(i) In green light:

• Appearance: The rose will appear black or very dark green.
• Reason: A red rose appears red in white light because its petals contain pigments that absorb most colors of light and reflect primarily red light. When only green light shines on the rose, there is no red light for the pigments to reflect. The green light will be absorbed by the pigments (though not as strongly as blue or other colors), and very little or no light will be reflected back to our eyes. Without reflected light, the rose will appear black (absence of color) or a very dark shade of the illuminating color (very dark green in this case, if there’s minimal reflection).

(ii) In red light:

• Appearance: The rose will appear red, possibly even a more vibrant red.
• Reason: When red light shines on a red rose, the pigments in the petals will readily reflect this red light. Since the rose’s pigments are efficient at reflecting red light (that’s why it looks red in white light), it will appear red when illuminated with red light. In fact, because there are no other colors present to be absorbed significantly, the reflected red might appear even more intense or saturated.

Question 32.

Why does a piece of paper appear white in sunlight ? How would you expect it to appear when viewed in red light?

Ans:

A piece of paper appears white in sunlight because its surface reflects all the colors of the visible spectrum in roughly equal proportions. Sunlight, as you know, is composed of all these colors. The fibers in white paper do not preferentially absorb any particular color, so all the colors are scattered back to our eyes, and our brain interprets this combination of all colors as white.  

Now, let’s consider how that same piece of white paper would appear when viewed in red light:

In red light, the paper would appear red. This is because the only color of light illuminating the paper is red. Since the paper doesn’t absorb red light (as it reflects all colors in white light), the red light will be reflected off its surface and travel to our eyes. As there are no other colors present in the illuminating light to be reflected, we will perceive the paper as the color of the light shining on it, which is red.

Question 33.

A piece of paper appears black in sunlight. What will be it£ colour when seen in red light ?

Ans:

Because a black piece of paper soaks up nearly all the colors present in light, when you shine a red light on it, the only light available to be reflected back to your eyes is red. Even though a black object is a poor reflector overall, whatever tiny amount of light it does reflect will be the color of the light source illuminating it. Therefore, the paper will appear red, albeit likely a very dark or muted red due to its overall absorptive nature.