Lakhmir Singh & Manjit Kaur: Light- 1 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.1. What is meant by ‘incident ray’?

A ray of light that strikes a surface or falls on a surface before being reflected, transmitted or absorbed is known as incident ray.

Q.2. What is meant by ‘reflected ray’? 

The light ray reflected by a surface, when an incident ray strikes the same surface is the reflected ray. The reflected ray corresponds to the incident ray always.

Q.3. How many reflected rays can there be for a given single incident ray falling on a plane mirror? 

For a single incident ray falling on a plane mirror, there is always a single reflected ray. The angle of incidence is always equal to the angle of reflection, this is by the law of reflection.

Q.4. What do you understand by the term ‘point of incidence’? 

The point of incidence is the point where the incident ray strikes the mirror.

Q.5. What is ‘normal’ in the reflection of light from a plane mirror? 

Normal is an imaginary line perpendicular to the surface, in the reflection of light from a plane mirror.

Q.6. Define the angle of incidence. 

The angle which incident ray makes with the perpendicular to the surface at the point of incidence.

Q.7. Define the angle of reflection. 

The angle between the reflected ray and the line perpendicular to the reflecting surface at the point of incidence is known as the angle of reflection.

Q.8. A ray of light is incident on a plane mirror at an angle of 30°. What is the angle of reflection? 

The angle of reflection will be 30 degree. This is by the law of reflection that angle of incidence is equal to the angle of reflection.

Q.9. An incident ray makes an angle of 75° with the surface of a plane mirror. What will be the angle of reflection?

The angle made by the normal will be 90 – 75 = 15. Angle made by normal is equal to angle of incidence. So i = 15 degree. And by the law of reflection which states that angle of incidence is equal to the angle of reflection, the angle of reflection will be 15 degree.

Q.10. A ray of light is incident normally (perpendicularly) on a plane mirror. Where will this ray of light go after reflection from the mirror? 

When a ray of light is incident normally (perpendicularly) on a plane mirror, the ray reflects and follows the same path.

Q.11. What is the angle of incidence when a ray of light is incident normally on a plane mirror? 

The angle of incidence when a ray of light is incident normally on a plane mirror is zero degree because the ray is incident normally.

Q.12. What is the angle of reflection when a ray of light is incident normally on a plane mirror? 

The angle of reflection when a ray of light is incident normally on a plane mirror will be zero degree because the angle of incidence will be zero degree as the ray is incident normally. By law of reflection, angle of incidence = angle of reflection.

Q.13. What is the angle of incidence of a ray of light if the reflected ray is at an angle of 90° to the incident ray?

The angle of incidence = angle of reflection.

According to the question, given i + r = 90 degree

i = r

hence i + i = 90 degree

2i = 90 degree.

I = 45 degree.

Therefore, the angle of incidence is 45 degree.

Q.14. Name the apparatus which is used to obtain a thin beam of light.

The apparatus used to obtain a thin beam of light is known as Ray box. The rays are bright enough to be viewed easily. A ray box is a standard source of light used in optics experiments. 

Q.15. What type of reflection of light takes place from:
(a) a rough surface?
(b) a smooth surface?

(a) Diffused reflection takes place from a rough surface. The reflected rays are not parallel in diffused reflection.
(b) Regular reflection takes place from a smooth surface. The reflected rays are parallel in regular reflection.

Q.16. Which type of reflection of light, regular reflection or diffuse reflection, leads to the formation of images?

Regular reflection, wherein the reflected rays are parallel to each other, lead to the formation of images because all the rays get reflected in a particular direction. Regular reflection always occurs from a smooth surface. Diffused reflection does not lead to the formation of images because the rays do not reflect in one particular direction.

Q.17. What type of reflection of light takes place from:
(a) a cinema screen?
(b) a plane mirror? 

(a) Regular reflection occurs from a cinema screen because the surface of the screen is smooth.
(b) Regular reflection occurs from a plane mirror because the surface of a plane mirror is smoot/ even.

Q.18. If an object is placed at a distance of 7.5 cm from a plane mirror, how far would it be from its image?

The object would be 15cm far away from its image if it is placed at a distance of 7.5cm from the plane mirror. The object is at 7.5cm from the mirror, then the image of the object is 7.5cm on the other side of the mirror. Hence, the image is a total of 15cm from object (7.5cm to the mirror + 7.5cm to the image).

Q.19. Is the image of an object in a plane mirror: virtual or real? 

The image of an object in a plane mirror is virtual. Virtual image cannot be projected onto the screen because it is not real.
A virtual image is a copy of the object, formed on the plane mirror. The light rays never come from the image. They appear to come from the object.

Q.20. Name the phenomenon responsible for the following effect: When we sit in front of a plane mirror and write with our right hand, it appears in the mirror that we are writing with the left hand. 

The phenomenon is known as Lateral inversion. The effect produced by a plane mirror in reversing the images from left to right and right to left while forming images is lateral inversion.

Q.21. Name a device which works on the reflection of reflected light. 

Periscope is one such device which works on the reflection of reflected light.
The light from the object falls on one mirror that is placed at 45^o to the object and the light gets reflected. This reflected light in turn falls on another mirror and is again reflected until it reaches the eyepiece.

Q.22. How are the two plane mirrors in a periscope arranged:
(a) with respect to one another?
(b) with respect to sides of the tube?

(a) The 2 plane mirrors are parallel to one another.
(b) The plane mirrors make an angle of 45 degrees with respect to the sides of the tube.

Q.23. What will be the number of images formed when an object is placed between two parallel plane mirrors facing each other? 

Infinite number of images are formed when an object is placed between two parallel plane mirrors facing each other. The object undergoes multiple reflection to produce image of the distant object. The distance between the mirror and the object is inconsiderable in formation of infinite images. No matter how much the distance be, number of images formed will be infinite.

Q.24. Name an instrument or toy which works by producing multiple reflections from three plane mirrors to form beautiful patterns. 

Kaleidoscope is the instrument that works by producing multiple reflections. It consists of 3 plain mirrors inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

Q.25. State one use of kaleidoscope. 

A Kaleidoscope is a toy mostly used by children for enjoyment. It is made of a tube with plain mirrors and colored glass pieces inside it at one of the end. Kaleidoscope having object chambers are used for hiding valuables.

Q.26. Name the device used to split white light into seven colours. 

A Glass Prism. It is a device used to split white light into seven colors. It has 2 triangular ends and 3 rectangular sides which help in splitting the white ray of light into a band of 7 colors, also known as spectrum.

Q.27. What happens when a beam of sunlight is passed through a glass prism? 

When a beam of sunlight is passed through a glass prism, it splits up into a band of 7 colors, also known as a spectrum.

Q.28. What type of lens (convex or concave) is present in the human eye? 

The human eye has convex type of lens. Light that hits the lens of the eye is focused by the lens so that it hits the retina. Retina has photoreceptor cells (rods and cones) which get stimulated and sends signals to the brain for viewing.

Q.29. What is the range of vision of a normal human eye? 

The range of vision of a normal human eye is from 25cm to infinity. The minimum distance for an object to be viewed clearly for a normal human eye is 25cm and the view may extend to infinity.

Q.30. Name the point inside the human eye where the image is not visible. 

Blind spot is the point in human eye where the image is not visible. This is because there are no photoreceptors, i.e., rods and cones present in the optical disk.

Q.31. Name the phenomenon which enables us to see movies in a cinema hall. 

The phenomenon which enables us to see movies in a cinema hall is known as Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.

Q.32. Name an eye ailment (or eye-disease) caused by the deficiency of vitamin A in the diet.

Deficiency of Vitamin A in the diet causes night blindness. Vitamin A helps in transforming the nerve impulses into images in the retina. Deficiency of vitamin A (also known as retinol) causes night blindness. Night blindness is the inability to see properly at night or in poor light.

Q.33. What is the name of transparent front part of an eye? 

Cornea is the transparent front part of an eye. It covers the front portion of the eye.

Q.34. What is the name of a small opening in the iris of an eye? 

A small opening in the iris of an eye is known as pupil. Pupil controls the amount of light that enters the eye.

Q.35. Which part of the eye gives it its distinctive color?

Iris, the part of our eye, contains pigment and gives the eye its distinctive color. Iris also helps in controlling the size of the pupil by constricting and dilating it, thus reducing and increasing the amount of light entering the eye.