Light Class 8 Worksheet Science Chapter 13

Q.1. What is angle of incidence?

The angle between the normal and incident ray is called the angle of incidence (∠i).

Q.2. Which part of the eye gives it its distinctive color?

The iris is the part of that eye which gives it its distinctive colour. When we say that a person has green eyes, we refer actually to the colour of the iris.

Q.3. What are rods and cones in the retina of an eye?

There are two kinds of light sensitive cells on the retina.
(i) cones, which are sensitive to bright light and
(ii) rods, which are sensitive to dim light.
Besides, cones sense colour.

Q.4. Explain why, an owl can see well in the night but not during the day whereas an eagle can see well during day but not in the night.

A night bird (owl) can see very well in the night but not during the day. On the other hand, day light birds (kite, eagle) can see well during the day but not in the night. The Owl has a large cornea and a large pupil to allow more light in its eye. Also, it has on its retina a large number of rods and only a few cones. The day birds on the other hand, have more cones and fewer rods.

Q.5. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper

(a) Polished wooden table: Regular reflection. A polished wooden table has smooth surface. Hence reflections from the polished table will be regular.

(b) Chalk powder: Diffused reflection. Chalk powder has irregular surface. Hence diffused reflections will take place from chalk powder.

(c) Cardboard surface: Diffused reflection. Cardboard surface has irregular surface. Hence diffused reflections will take place from Cardboard surface.

(d) Marble floor with water spread over it: Regular reflection. Marble floor with water spread over it is an example of regular surface. Hence regular reflection will take place.

(e) Mirror: Regular reflection. Mirror has smooth surface. Hence regular reflection will take place.

(f) Piece of paper: Diffused reflection. Although a piece of paper may look smooth, but it has many irregularities on its surface. Hence diffused reflections will take place from paper.

Q.6. What are the main parts of the human eye?

Important parts of the eye are cornea, iris, pupil, lens, retina and optic nerve.

Q.7. A ray of light is incident on a plane mirror at an angle of 40°. What is the angle of reflection?

The angle of reflection will be 40 degree. This is by the law of reflection that angle of incidence is equal to the angle of reflection.

Q.8. How do we see a moving picture or a movie?

The movies that we see are actually a number of separate pictures in proper sequence. They are made to move across the eye usually at the rate of 24 pictures per second (faster than 16 per second). So, we see a moving picture.

Q.9. How does eye adjust itself to deal with light of varying intensity?

The iris controls the amount of light entering into the eye by automatically adjusting the size of the pupil according to the intensity of the light that the eye receives. If the amount of light is high, the iris contracts the pupil and reduces the amount of light entering the eyes. If the amount of light is less, the iris expands the pupil so that more light can enter the eye and the things can be viewed clearly.

Q.10. What are the functions of the following parts of the eye?
Iris, Eye-lens, Retina and Optic nerve

• Iris: Iris controls the size of the pupil. The iris is the part of that eye which gives it its distinctive colour.
• Eye-lens: The lens focuses light on the back of the eye, on a layer called retina.
• Retina: Retina receives the light focused by the lens. Retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain.
• Optic nerve: The job of the optic nerve is to transfer visual information from the retina to the brain.

Q.11. Describe the construction of a kaleidoscope.

To make a kaleidoscope, get three rectangular mirror strips about 15 cm long and 4 cm wide each. Join them together to form a prism. Fix them in a circular cardboard tube or tube of a thick chart paper. Make sure that the tube is slightly longer than the mirror strips. Close one end of the tube by a cardboard disc having a hole in the centre, through which we can see. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate. Place on this glass plate several small pieces of coloured glass (broken pieces of coloured bangles). Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around. Kaleidoscope is ready. When we peep through the hole, we will be able to see a variety of patterns in the tube.

Q.12. What type of reflection of light takes place from a rough surface?

The diffuse reflection of light takes place from a rough surface.

Q.13. What is an 'incident ray'?

The light ray, which strikes any surface, is called the incident ray.

Q.14. How many mirrors are there in a periscope?

The periscope makes use of two plane mirrors.

Q.15. What is meant by dispersion of light'? Name a natural phenomenon showing dispersion.

Splitting of light into its colours is known as dispersion of light. Rainbow is a natural phenomenon showing dispersion.