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 types of light-sensitive cells in the retina:

• Rods: These cells are sensitive to dim light.
• Cones: These cells respond to bright light and are responsible for colour vision.

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.

Owls are excellent at seeing in the dark, while eagles are adapted for daytime vision. Here’s why:

• Owls have a large cornea and pupil that let in more light.
• They possess many rods in their retina, which are sensitive to low light.
• Owls have fewer cones, which are responsible for colour vision.

In contrast:

• Eagles have more cones, allowing them to see well in bright light.
• They have fewer rods, making them less effective in dim conditions.

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?

Main parts of the human eye include:

• Cornea - The clear front layer that protects the eye.
• Iris - The coloured part that controls the size of the pupil.
• Pupil - The opening that allows light to enter the eye.
• Lens - Focuses light onto the retina.
• Retina - The layer at the back of the eye that detects light.
• Optic nerve - Transmits visual information to the brain.

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 is 40 degrees. This follows the law of reflection, which states that the 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 regulates the amount of light that enters the eye by adjusting the size of the pupil. This process occurs automatically based on light intensity:

• In bright light, the iris contracts the pupil, reducing light entry.
• In dim light, the iris expands the pupil, allowing more light to enter for clearer vision.

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 construct a kaleidoscope, follow these steps:

• Obtain three rectangular mirror strips, each measuring about 15 cm long and 4 cm wide.
• Join the mirror strips to form a prism.
• Secure this prism inside a circular cardboard tube or a thick chart paper tube. Ensure the tube is slightly longer than the mirror strips.
• Close one end of the tube with a cardboard disc that has a hole in the centre for viewing.
• To enhance durability, attach a piece of transparent plastic sheet beneath the cardboard disc.
• At the opposite end, place a circular plane glass plate that touches the mirrors.
• On this glass plate, add several small pieces of coloured glass, such as broken pieces of coloured bangles.
• Seal this end of the tube with a ground glass plate, allowing space for the coloured pieces to move freely.

Your kaleidoscope is now ready! When you look through the hole, you will see a variety of patterns. An interesting aspect of a kaleidoscope is that you will never see the same pattern twice. Artists and designers often use kaleidoscopes for inspiration in creating new patterns. To make your kaleidoscope more attractive, consider wrapping it in coloured paper.

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

Reflection of light from a rough surface is known as diffused reflection. This occurs when:

• The surface is irregular, causing light rays to scatter in different directions.
• Unlike a smooth surface, where light reflects uniformly (known as regular reflection), rough surfaces do not maintain parallelism in reflected rays.
• This scattering allows us to see objects from various angles, as the light is reflected in multiple directions.

In summary, diffused reflection is essential for visibility in everyday life, as it enables us to see objects that do not emit their own light.

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.

Dispersion of light refers to the process where light splits into its constituent colours. This occurs when light passes through a medium, such as a prism, causing it to bend and separate into different wavelengths. A well-known natural phenomenon that demonstrates dispersion is the rainbow. Rainbows form when sunlight passes through raindrops, resulting in a spectrum of colours appearing in the sky.