Class 8 Science - Question Answers Light

Q1: Define light. Discuss its importance.

Ans: Light is an electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has a wavelength in the range of about 380 nanometres to about 740 nm  between the invisible infrared.
Q2: Why we are not able to see any object in the dark?

Ans: We can see any object when light reflected by that object reaches our eyes, but in dark room no light is reflected by the object thus we are not able to see in the dark.
Q3: Why the image formed by concave mirror is sometimes real, while sometimes virtual?

Ans: In case of concave mirror the image depends upon the distance of the object from the mirror. If the object is beyond the focus then real image is formed and if the object is closer than the focus then virtual image is formed
Q4: Why convex mirrors are used in vehicles?

Ans: Convex mirrors are used in vehicles because the image formed by convex mirror is always erect and smaller in size.
Q5: Explain the two laws of reflection.

Ans: There are two laws of reflection: the angle of incidence is equal to the angle of reflection and incident rays, reflected rays and normal rays drawn at the point of incidence to the reflecting surface lies in the same plane.

Q6: What kind of image is formed by a plane mirror?

Ans: Image formed by plane mirror is virtual, upright and of the same shape and size as of the object.
Q7: What do you mean by lateral inversion?

Ans: Lateral inversion is the reversal experienced by the image formed in a flat mirror. Although the image is the correct way up, its left and right sides are transposed.
Q8: "Reflected light can be reflected again". Give an example to justify this statement.

Ans: Sit in front of a mirror, tell your friend to hold a mirror behind you to see your hair cut, your hair image will be shown in the mirror in front of you, this is the best example of reflected light can be reflected again.
Q9: Explain all parts of a human eye.

Ans: The eye has roughly spherical surface. The outer coat of eyes is white and tough to protect the interior of eyes from any kind of accident. Its transparent front part is called cornea, behind the cornea there is a dark muscular structure called iris. In the iris there is small opening called pupil. The size of the pupil is controlled by the iris, the iris control the amount of light entering into the eye.
Q10: Explain the function of retina of eyes.

Ans: The lens focuses light on the retina which contains several nerve cells. Sensations felt by the nerve cells are then transmitted to brain through the optic nerve.
Q11: Why blind spot is named so?

Ans: At the junction of the optic nerve and the retina, there are no sensory cells, so there is no possibility of vision at that spot, thus it is called blind spot.
Q12: What is the function of eyelids?

Ans: Eyelids prevent any object from entering the eye, eyelids also shut out light when not required.
Q13: What is myopia? How it can be corrected?

Ans: Myopia is a vision condition in which close objects are seen clearly, but objects farther away appear blurred. Most commonly myopia can be corrected through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, though there are cases of associated side effects. The corrective lenses have a negative optical power (i.e. are concave)
Q14: What is hyper myopia? How it can be corrected?

Ans: Hypermetropia means long sight and is where the image of a nearby object is formed behind the retina. This could be because the eye is too short, or the cornea or crystalline lens does not refract the light enough. A hypermetropic person may have blurred vision when looking at objects close to them, and clearer vision when looking at objects in the distance. By placing a convex (plus powered) lens in front of a hypermetropic eye, the image is moved forward and focuses correctly on the retina.
Q15: Why eyesight becomes foggy in old age?

Ans: In old age eyes eye lens becomes cloudy, and person develops a cataract that is loss of vision. This defect can be treated by replacing opaque lens with a new artificial lens.
Q16: How cataract disorder can be corrected?

Ans: In old age eyes eye lens becomes cloudy, and person develops a cataract that is loss of vision. This defect can be treated by replacing opaque lens with a new artificial lens.
Q17: Give some possible measures to take care of the eyes.

Ans: Some possible measures are as follows:
1. If advised use suitable spectacles

2. Avoid too much or too little eyes

3. Do not look at the sun or a powerful light directly

4. Never rub your eyes

5. Wash your eyes properly and frequently with water

6. Always read at the normal distance for vision.
Q18: What is the braille system?

Ans: Braille is a writing system which enables blind and partially sighted people to read and write through touch. It was invented by Louis Braille.
Q19: Why owl can see very well in the night but not during the day like other animals?

Ans: The owl has a large cornea and a large pupil to allow more light into the eye. It has large number of rods on its retina and few cones that enables it to see properly in dim light or in dark.
Q20: Differentiate between regular and irregular reflection. Does irregular reflection mean the failure of the laws of reflection?

Ans: 

In diffused reflection each ray obeys the law of reflection, thus there is no failure of law of reflection.

Q21: Mention whether diffused or regular reflection will take place when :

1. A beam of light strikes a mirror
2. A beam of light strikes a cardboard surface.
3. A beam of light strikes a piece of paper. To the incident ray

Ans: 

1. Regular reflection because mirror has smooth surface
2. Diffused reflection because cardboard has irregular surface.
3. Diffused reflection, because a piece of paper appears smooth but it has lot of minor irregularities.

Q22: Give an important characteristic of a normal eye.

Ans: Normal eye can clearly see distant objects as well as object nearby.

Q23: What is reflection?
Ans. When rays of light return at some angle by striking a smooth surface, this phenomenon is called reflection of light.

Q24: What is incident ray?
Ans. The light ray, which strikes any surface is called the incident ray. 

Q25: What is reflected ray?
Ans. The ray that comes back from the surface after reflection is known as the reflected ray.

Q26: Define normal.
Ans. A perpendicular line on the mirror at the point where incident ray strikes is called normal.

Q27: Define angle of incidence.
Ans. The angle between the normal and incident ray is called the angle of incidence.

Q28: Define angle of reflection.
Ans. The angle between the normal and reflected ray is called angle of reflection.q 

Q29: Write the laws of reflection.
 Ans. There are two laws of reflection.
(i) Angle of incidence is equal to angle of reflection.
(ii) Incident ray, reflected ray and normal at the point of incidence, all lie in the same plane.

Q30: What is lateral inversion?
Ans. When an image is formed by a mirror the left of the object appears on the right and the right appears on the left, this is known as lateral inversion.

Q31: What do you mean by diffused or irregular reflection?
Ans. When all the parallel rays reflected from a plane surface are not parallel, the reflection  is known as diffused or irregular reflection.

Q32: Define regular reflection.
Ans. When reflected rays from a smooth surface are parallel, it is known as regular reflection.

Q33: What do you mean by multiple images?
 Ans. When two mirrors are kept parallel to each other then numerous images of an object are seen in these mirrors. This is known as multiple images.

Q34: What is the use of kaleidoscope?
Ans. Designers of wallpapers, fabrics, artists use kaleidoscope to get ideas for new patterns. 

Q35: Define dispersion of light.
Ans. Splitting of light into its seven colours is known as dispersion of light.

Q36: Give an example of natural dispersion.
Ans. Rainbow is a natural phenomenon showing dispersion. 

Q37: What is the function of rods and cones in our eye?
Ans. Cones are sensitive to bright light and colour whereas rods are sensitive to dim light.
Q38: What is blind spot?
Ans. At the junction of the optic nerve and retina, there are no sensory cells, so no vision is possible at the spot. This is called the blind spot.

Q39: What is the function of eyelids.
Ans. Eyelids prevent any object from entering the eye. They also shut out light when not required. 

Q40: What is the Braille system?
Ans. Louis Braille developed a system for visually challenged persons. This is known as Braille system. He developed Braille code for common languages.

Q41: How many images of a mobile will be formed if it is placed between two parallel plane mirrors separated by 30 cm?

Ans: Infinite number of image will be formed if it is placed between two parallel plane mirrors .