Refraction and Dispersion of Light: 8 CBSE

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.