Refraction and Dispersion of Light: 8 CBSE

Question: What do you mean by refraction of light? In which direction will light bend when travelling from an optically rarer medium to an optically denser medium? Answer: The bending of light as it passes from one transparent medium to another is called refraction. It occurs because light travels at lower speed in an optically denser medium. Light bends towards the normal at the point of incidence as it passes from an optically rarer to an optically denser medium. Question: Why does refraction occur? Answer: Refraction occurs because of the difference in speed of light in different mediums. Question: What is dispersion? Answer: The splitting of light into its constituent colors is called dispersion. Question: Which medium will bend light more - glass or water? Why? Answer: Light will bend more in glass than in water as glass is optically denser than water. Question: What is 'white light'? Answer: Sunlight is a mixture of light of several colors. It is known as white light. Question: What is the order of colors in a spectrum of 'white light'? Answer: Red, orange, yellow, green, blue, indigo and violet. Question: Distinguish between near point and far point of the eye. Answer: The least distance at which the eye can see an object clearly is called the near point whereas the far point of the eye is the maximum distance at which it can see clearly. Question: What is the 'blind spot' of the eye? Answer: Blind spot is the region of the retina at the junction of the optic nerve and the retina. There are no sensory cells at this spot, hence any image formed here cannot be seen. Question: What is the difference between the displacement produced by a rectangular glass slab in a ray of light and the deviation produced by a glass prism? Answer: Question: Explain the term 'spectrum'. Why does a glass prism split white light into its constituent colours? Answer: The splitting up of white light into its constituent colors is called dispersion. The patch of colored light obtained due to dispersion through a glass prism on the screen is known as a spectrum. A glass prism splits white light into its constituent colors because of deviation. Different colors of light are deviated by different amounts. Hence the colors separate out on emerging from the prism. Question: Distinguish between converging and diverging lenses. Answer: Question: Define the following terms for a lens: a. optical centre b. principal axis c. principal focus d. focal length Answer: Optical centre: The optical centre of a lens is a point on the principal axis such that a ray passing through it does not undergo any deviation. Principal axis: A straight line passing through the centres of curvature C1 and C2 of the two surfaces of the lens is called the principal axis. Principal focus: If a beam of light parallel to the principal axis is incident on a converging lens, the rays of light after passing through the lens converge at a point F. This point is called the principal focus or simply the focus of the lens. Focal length: The distance between the optical centre and the focus of a lens is called the focal length of the lens. Question: Why does a concave lens always form a virtual image of an object? Draw a diagram to illustrate this. Answer: Image formed by a concave lens: The rays falling on a concave lens, after refraction, always diverge. Therefore, no matter where the object is kept, a diverging lens always forms a virtual image, which is erect and smaller in size than the object. It is formed between the optical centre and the focus, on the same side of the lens as the object. Question: What do you mean by the accommodation of the eye? What gives the eye the power of accommodation? Answer: The ability of the eye to alter the focal length of its lens so that it can clearly see all objects within a certain range (25 cm, near point to infinity far point) is called accommodation of the eye. Question: How are letters written in the Braille system? Give two examples. Answer: The Braille system is a method of writing for the blind. It employs groups of raised dots to represent printed letters and numbers. It was invented by Louis Braille of France in the early 1800s. It is not a separate language, but just another way to read and write English or any other language. Every character in the Braille code is based on an arrangement of one to six raised dots. Each dot has a numbered position in the Braille cell. These characters make up the letters of the alphabet, punctuation marks, numbers, and also special characters. For example: The letter 'A' is written with only 1 dot. and The letter 'D' has 1, 4, and 5. Question: What is meant by 'persistence of vision'? How is this property of the eye used in movie films? Answer: When an image of an object is formed on the retina, it stays for about 1/16th of a second. This is called persistence of vision. Moving images that we see on the TV or in a cinema depend on this property of the eye. If successive still images of any moving object, person or animal are flashed before our eyes at the rate of more than 16 images per second, we would see it as a movie of running of that object, person or animal. Question: A coin kept at the bottom of a glass bowl full of water appears to be higher than it actually is. With the help of a labelled diagram, explain why this happens. Answer: A coin kept at the bottom of a glass bowl full of water appears to be higher than it actually is. This happens because rays of light from any point (say A) on the coin, refract away from the normal as they emerge from water into air. The rays appear to come from a point A' above A. Thus, A' is the virtual image of A. In the same way, each point on the coin has a corresponding virtual image above the original point. Thus, the coin appears to be higher than it actually is. Question: Draw a labelled diagram of the eye and answer the following questions: a. From where does light enter inside the eye? b. Which muscles control the focal length of the eye lens? c. Where is the image formed? d. Which nerve sends the image signals to the brain? Answer: (a) Light enters the eye through the cornea. (b) Ciliary muscles control the focal length of the eye lens. (c) Image is formed on the retina. (d) Optic nerve sends the image to the brain. Question: Distinguish between myopia and hypermetropia. Draw diagrams to show how these defects can be corrected by using suitable lenses. Answer: Question: Suppose a swimming pool is filled with an imaginary liquid which is optically rarer than air. For a person looking at the pool from outside, will the apparent depth of the pool be less than or greater than the real depth? Give reasons. Answer: The apparent depth of the pool will be greater than the real depth if the swimming pool is filled with an imaginary liquid which is optically rarer than air. This will happen because rays of light from any point of the floor of the pool refract towards the normal as they emerge from a rarer to a denser medium. Question: What type of image is formed when rays of light coming from a point on the object diverge after reflection or refraction—real or virtual? Answer: When rays of light coming from a point on the object diverge after reflection or refraction, a virtual image is formed. Question: Will a spectrum be formed if blue light falls on a prism? Give reasons. Answer: No. It is a light of single colour. So it will not be spitted up to form a spectrum.

Class Notes NCERT Solutions for CBSE Students

Refraction and Dispersion of Light: 8 CBSE

Question: What do you mean by refraction of light? In which direction will light bend when travelling from an optically rarer medium to an optically denser medium?

Question: Why does refraction occur?

Question: What is dispersion?

Question: Which medium will bend light more – glass or water? Why?

Question: What is ‘white light’?

Question: What is the order of colors in a spectrum of ‘white light’?

Question: Distinguish between near point and far point of the eye.

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