Light Class 7 Notes Science Chapter 13

Table of Contents
1. What is light?
2. Light travels along a straight line
3. Reflection of light
4. Image and object
5. Playing with spherical mirrors
6. Types of images
7. Images formed by lenses
8. Sunlight - white or coloured?
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Have you noticed sunlight shining into a room through a small hole, or the bright lights from a car at night? These beams of light help us see and show how light travels. Let us learn more about how light works and why it is important.

What is light?

Light is a form of energy that enables us to see the world around us. It travels in straight lines and can pass through different media such as air, water and glass. Light carries information about the shape, colour and position of objects.

Light travels along a straight line

This property of light can be demonstrated by the candle-and-pipe experiment.

1. Fix a candle at one end of a straight pipe and look through the other end. The light from the candle travels straight through the pipe and you can see the flame clearly.

2. Bend the pipe and try to look through it again. The light from the candle now falls on the sides of the pipe and cannot reach your eyes, so the flame cannot be seen.

Conclusion: light travels in straight lines; if its path is blocked or bent, the object cannot be seen along that path.

Reflection of light

When light falls on a smooth and shiny surface, it changes its direction. This change of direction of light at a surface is called reflection.

• Shiny surfaces such as a steel plate, the surface of water, or a spoon reflect light.
• You can often see trees, buildings or the sky reflected on calm water.
• A plane mirror reflects light according to the angle at which incident light strikes it; this determines the direction of the reflected rays.

Reflection of light from a mirror

Image and object

• Place a candle in front of a plane mirror and look at the flame. You will see another flame appearing to be behind the mirror; this is called the image.
• The actual candle in front of the mirror is called the object.
• Move the candle to different positions in front of the mirror and observe how the image changes position accordingly.
• If the distance between the object and the mirror increases, the distance between the image and the mirror also increases.
• The size of the image may change when the object-mirror distance changes: increasing the object-mirror distance generally makes the image smaller, and decreasing that distance generally makes the image larger.

Candle in front of a plain mirror

Erect: An image is said to be erect if it is formed the same way up as the object.

Inverted: An image is called inverted if it is formed upside down compared to the object.

Inverted image of a candle

Right or left?

When you stand in front of a mirror and raise your left hand, it appears as if the person in the mirror is raising their right hand. If you touch your right ear, it looks as if the person in the mirror is touching their left ear. This apparent interchange of left and right is a property of mirror images; the mirror reverses the front-back direction, and this is seen as a left-right reversal by the observer.

Left- Right Inversion

Interesting fact: The word "AMBULANCE" is often painted on the front of ambulances in reversed (mirror-image) letters so that drivers ahead can read it correctly in their rear-view mirrors.

Left-Right Reversal of Images 

Playing with spherical mirrors

Spherical mirrors are parts of the surface of a sphere. They are of two kinds depending on which side of the sphere is reflective.

Concave mirror

• If the reflecting surface is toward the centre of the sphere (curves inward), the mirror is called a concave mirror.
• A concave mirror looks like the inner surface of a spoon and can focus parallel rays of light to a point. It can form magnified or diminished images, and can produce real or virtual images depending on the object position.

Convex mirror

• If the reflecting surface is outward (curves outward), the mirror is called a convex mirror.
• A convex mirror looks like the back of a spoon. It diverges light rays and always forms a virtual, upright and smaller image, but provides a wider field of view.

Where you see them:

• Concave mirrors are used in torches/headlights, shaving or make-up mirrors (to get a magnified virtual image), and some reflecting telescopes (which use a concave primary mirror).
• Convex mirrors are used as rear-view mirrors and side mirrors in vehicles because they show a wide area behind the vehicle.

Types of Mirrors

Types of images

• Real image: An image that can be formed on a screen is called a real image. A real image is formed due to actual convergence of light rays. For example, the image formed on the screen by a camera lens (on the photographic film or sensor) is real.
• Virtual image: An image that cannot be obtained on a screen and appears to be behind the mirror or lens is called a virtual image. The image in a plane mirror is virtual.

Real and Virtual Image

Images formed by lenses

A lens is a transparent piece of glass or plastic that refracts (bends) light rays. Lenses are used in spectacles, microscopes, telescopes, cameras and many other instruments.

Convex lens

• A convex lens is thicker at the centre than at the edges and is also called a converging lens.
• Parallel rays of light passing through a convex lens converge to a point known as the focus. Depending on the object position, a convex lens can form real inverted images or virtual erect images (magnified or diminished).

Concave lens

• A concave lens is thinner at the centre and thicker at the edges and is also called a diverging lens.
• Light rays passing through a concave lens diverge and appear to come from a point; a concave lens always forms a virtual, erect and smaller image than the object.

Lens

Converging and Diverging Lens

Applications of lenses

• Convex lenses are used in magnifying glasses, cameras, microscopes, binoculars and the objective/eyepiece lenses of refracting telescopes.
• Concave lenses are used in some types of spectacles (for short-sightedness) and in devices where a wider field of view or image reduction is needed.
• A reflecting telescope uses a concave mirror as the primary element; a refracting telescope uses convex lenses.

Image by a convex lens for object placed at different distance from it
Virtual image by convex lens          Image by a concave lens

CAUTION

Do not look at the Sun or any very bright source of light directly through a lens. Do not focus sunlight with a convex lens on your skin or on flammable material.

Activity instructions

1. Take a convex lens (magnifying glass) and place it in the path of sunlight in a darkened room or in the open.
2. Put a sheet of paper at the other side of the lens and adjust the distance between the lens and the paper until you see a bright spot of light on the paper.
3. Hold the lens steady for a few minutes and observe whether the paper begins to char or burn (exercise great caution and do this under adult supervision).
4. Now replace the convex lens with a concave lens and try the same. You will not see a bright focused spot because a concave lens diverges light rays.

Similar to mirrors, the position of an object with respect to a lens affects the nature, size and position of the image.

In general, a convex lens can produce a real, inverted image (when the object is beyond the focal length) or a virtual, erect and magnified image (when the object is within the focal length). A concave lens always produces an erect, virtual and smaller image than the object.

Sunlight - white or coloured?

A rainbow is a band of colours that appears when sunlight falls on raindrops. The appearance of a rainbow shows that sunlight is made up of many colours mixed together.

Rainbow

A rainbow is formed when sunlight is refracted, reflected and dispersed by water droplets in the atmosphere. It appears as an arc showing seven basic colours: red, orange, yellow, green, blue, indigo and violet. This demonstrates that white sunlight is composed of coloured light of different wavelengths.

When light falls on thin films such as soap bubbles or on the surface of CDs, we see different colours. These colours arise because different wavelengths of light are reflected or interfered with differently, making the component colours visible.

Prism activity - to separate sunlight into colours

1. Take a glass prism in a darkened room.
2. Allow a narrow beam of sunlight to enter the room through a small hole and fall on one face of the prism.
3. When the beam passes through the prism, it bends and the different colours separate because each colour bends by a slightly different amount.
4. Allow the emergent light to fall on a white sheet of paper or a white wall. You will see the component colours spread out and visible as a spectrum.
5. This shows that sunlight is made up of many colours and that white light is a mixture of these coloured lights.

A prism splits a beam of sunlight into seven colors 

Newton's disc

Newton's disc is a disc divided into sectors painted with the seven colours of the rainbow. When spun very fast in daylight, the colours blend and the disc appears whitish. This is another demonstration that white light is a mixture of the component colours.

Newton's Disc