Class 8 Science

# Light

## Extra Questions

### Short Answer Type

Question 1: How does light enable us to see something?

Answer: When light falls on an object, some of the light is reflected back to our eyes. Thus, we are able to see an object because of light being reflected from the object.

Question 2: Write the two laws of reflection.

Answer: Laws of reflection are as follows:

First Law of Reflection: Angle of reflection is equal to angle of incidence.

Second Law of Reflection: The incident ray, reflected ray and normal at the point of incidence; all lie in the same plane.

Question 3: What do you understand by regular reflection?

Answer: When all the reflected rays are parallel to each other; this case of reflection is called regular reflection. Regular reflection happens from a smooth surface, e.g. a mirror. We get to see clear images when image is formed by regular reflection.

Question 4: What do you understand by irregular reflection?

Answer: When the reflected rays are not parallel to each other, this case of reflection is called irregular reflection. Irregular reflection happens from a rough surface. The laws of reflection are obeyed in this case as well but because of the irregularities in the surface, the reflected rays are not parallel. We get to see somewhat blurred images when image is formed by irregular reflection. The relative clarity of an image depends on the relative smoothness of the reflecting surface.

Question 5: Write a short note on multiple reflections?

Answer: When two or more mirrors are placed at some angles to each other, we get to see multiple images. Let us take an example in which two mirrors are placed opposite to each other. If an object is placed between them, its image is formed in both the mirrors. The image in one mirror would act as an object for another mirror and this sequence would continue. This will result in the formation of multiple images.

Question 6: What is blind spot?

Answer: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.

Question 7: Explain the formation of image in eyes.

Answer: Light rays enter the eye through pupil and pass through lens. Lens focuses light rays on retina, and makes real, inverted and smaller image on retina. Signals from retina are carried by optic nerve to the brain. Thus, we are able to see an object.

Question 8: What are the common vision defects?

Answer: Myopia, Hypermetropia, Cataract and Presbyopia

Question 9: Write a brief note on Braille system.

Answer: The Braille scripts are written in the form of raised dots. This system is composed of 63 characters. Each character is written on a grid of 6 cells. This can be written with the help of Braille slate and stylus. A reader needs to touch the Braille script to read it.

### Long Answer Type Questions

Question 1: How will you trace image formation in plain mirror?

Answer: Image Formation in Plane Mirror

In this figure, PQ is a plain mirror and O is an object. OA and OC are the incident rays; coming from the object. The incident rays are reflected from the mirror and reflected rays emerge as AB and CD. If AB and CD are extended behind the mirror, they appear to meet at point ‘I’. This is the point where image is formed. If OM is object distance and MI is image distance, then in plain mirror, OM = MI. This means image distance is equal to object distance in case of a plain mirror.

Question 2: Explain the structure of human eye.

Answer: The human eye has following main parts:

Cornea: The eyeball is covered with a tough layer. This layer is transparent on the front. This transparent portion is called cornea.

Iris: Iris is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.

Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil. When the light is bright, iris contracts and thus allows less light into the eye. When the light is dim, iris dilates and thus allows more light into the eye.

Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of an object.

Retina: Retina is at the back of the eye and marks the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types, which are as follows:

• Cone Cells: The cone cells are sensitive to bright light. They also give the sense of colour.
• Rod Cells: The rod cells are sensitive to dim light.

Optic Nerve: The optic nerve emerges from the back of the eye ball. This nerve goes to the brain.

Blind Spot: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.

Question 3: Suggest some steps for care of eyes.

Answer: Some tips for care of eyes are as follows:

• Do not read in too bright or too dim light.
• Do not look directly at a bright object or at the sun.
• Do not keep the book too close to your eyes, while reading. Don’t keep the book too far either.
• If something gets into the eye, do not rub the eye. Wash it with cold water.
• In case of any problem (like itching or burning sensation) consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.