CBSE Important Questions Class 8 Science Chapter 10 – Light: Mirrors and Lenses

Class 8 Science Chapter 10 Light: Mirrors and Lenses is part of the new NCERT Curiosity textbook. It explains that light travels in a straight line, how it is reflected by plane, concave and convex mirrors, and how lenses bend light. Students learn the laws of reflection, the nature of the image formed by a plane mirror, lateral inversion, how a periscope and a kaleidoscope work, and where concave and convex mirrors and lenses are used in daily life.

These CBSE important questions for Class 8 Science Chapter 10 cover why a dentist uses a concave mirror, the image formed by a plane mirror, why AMBULANCE is written in mirror writing, the mirrors used in a periscope, the uses of lenses, the shape of a concave lens, and why light cannot be seen through a bent pipe. Every question has a clear step-by-step answer in simple English, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. A dentist uses a concave mirror to examine a patient's teeth. Explain the principle behind using a concave mirror for this purpose, detailing the characteristics of the image formed.

Answer: A dentist holds the concave mirror close to the tooth, that is, within its focal length.
When an object is placed between the pole and the focus of a concave mirror, the image formed is virtual, erect and magnified.
Because the image is enlarged, the dentist can clearly see small cavities or cracks inside the mouth, and because it is erect, it is easy to understand what is being seen. The concave mirror also helps to focus light onto the tooth, which makes the area brighter.

Q2. The image formed by a plane mirror will be
(a) larger
(b) erect
(c) inverted
(d) tilted

Answer: (b) erect
A plane mirror always forms an image that is virtual, erect and exactly the same size as the object. It is formed as far behind the mirror as the object is in front of it, and it is laterally inverted (left and right sides are interchanged).

Q3. The height of a candle is 10 cm. The height of its image seen in a plane mirror is
(a) 20 cm
(b) 5 cm
(c) 10 cm
(d) 1 m

Answer: (c) 10 cm
A plane mirror never changes the size of the image. The image is always exactly the same size as the object, so a 10 cm candle gives a 10 cm image, no matter how far the candle is kept from the mirror.

Q4. The word 'AMBULANCE' is written in the
(a) reverse order on an ambulance
(b) form of its mirror image on an ambulance van
(c) correct order on an ambulance
(d) random order on an ambulance

Answer: (b) form of its mirror image on an ambulance van
The word is deliberately printed as its mirror image. A driver ahead of the ambulance sees it in the rear-view mirror of his own vehicle, and the mirror reverses it once more, so the driver reads 'AMBULANCE' the right way round and can quickly give way. This happens because of lateral inversion.

Q5. Due to reflection from a mirror, the direction of light
(a) turns in the reverse direction
(b) remains the same
(c) changes
(d) turns 90 degrees in the clockwise direction

Answer: (c) changes
When light falls on a mirror it bounces back, and its direction changes. By how much it changes depends on the angle at which the light strikes the mirror, because the angle of reflection is always equal to the angle of incidence. So it is not always a reverse turn or a fixed 90 degrees.

Q6. Periscopes make use of
(a) convex mirrors
(b) concave mirrors
(c) plane mirrors
(d) convex lenses

Answer: (c) plane mirrors
A periscope has two plane mirrors fixed parallel to each other at 45 degrees at the two ends of a long tube. Light from the object strikes the top mirror, is reflected down the tube to the lower mirror, and is reflected again into the eye. Plane mirrors are used because they give an image of the same size without any distortion. Periscopes are used in submarines, tanks and to see over a crowd.

Q7. Lenses are used
(a) by barbers for shaving
(b) for focusing light in a torch
(c) in automobiles for rear view
(d) as a magnifying glass by watch repairers

Answer: (d) as a magnifying glass by watch repairers
A convex lens held close to a tiny object forms a magnified, erect, virtual image, which is just what a watch repairer needs to see small parts. The other three options use mirrors: a plane mirror for shaving, a concave mirror in a torch, and a convex mirror for the rear view in vehicles.

Q8. The image formed when an object is placed before a concave lens is
(a) real or virtual
(b) always inverted
(c) always erect
(d) always real

Answer: (c) always erect
A concave lens spreads out (diverges) the light that passes through it, so the rays never actually meet. The image is therefore always virtual, erect and smaller than the object, wherever the object is placed.

Q9. A dentist uses
(a) a convex mirror
(b) a plane mirror
(c) a concave lens
(d) a concave mirror

Answer: (d) a concave mirror
Held close to the tooth, a concave mirror gives an enlarged, erect image, so the dentist can see small details clearly. A convex mirror would make the image smaller, and a plane mirror would keep it the same size.

Q10. For a concave lens, its middle portion is
(a) thicker than its edges
(b) of the same thickness as its edges
(c) thinner than its edges
(d) thinner than one edge but thicker than the other

Answer: (c) thinner than its edges
A concave lens curves inwards, so it is thin in the middle and thick at the edges. Because of this shape it spreads light rays apart, which is why it is also called a diverging lens. A convex lens is the opposite: thick in the middle and thin at the edges.

Q11. You cannot see the flame of a lighted candle through a bent pipe but you can see it through a straight pipe, because
(a) light travels in a straight line
(b) the pipe is polished
(c) the bent pipe was blocked
(d) the straight pipe is transparent

Answer: (a) light travels in a straight line
This property is called the rectilinear propagation of light. In a bent pipe the light from the flame cannot turn round the bend to reach the eye, so the flame is not seen. In a straight pipe the light travels straight to the eye, so the flame is visible.

Q12. The change in direction of light after striking a mirror is called
(a) a beam of light
(b) a searchlight
(c) reflection of light
(d) path of light

Answer: (c) reflection of light
When light falls on a smooth shiny surface such as a mirror, it bounces back into the same medium and its direction changes. This bouncing back is called reflection, and it follows two laws: the angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal all lie in the same plane.

Related Study Material on Extramarks

Q.1 A body A is positively charged while another body B is negatively charged. Then, there will be

A. force of attraction between A and B

B. force of repulsion between A and B

C. no force between A and B

D. maximum force of repulsion between A and B

Marks:1
Ans

force of attraction between A and B

Q.2 We can easily charge non-metals like rubber, woollen clothes, plastics, etc. whereas we cannot charge a copper rod by rubbing easily. Why

Marks:2
Ans

When the metallic rods like copper rod are rubbed, charges does not build on their surface because charges can escape through metals as they are conductors whereas when non-metals like rubber, woollen clothes, plastics, etc. are rubbed, charges will build up on their surface because charges are not conducted through them as they are insulators.

Q.3 What is earthing Why earthing is provided in buildings

Marks:3
Ans

The process of transferring of charge from a charged object to the earth is called earthing. Earthing is provided in buildings to protect them from electrical shocks due to any leakage of electrical current. For our safety, most of the electrical appliances and the mains of the house are connected to earth, so that we can be prevented from getting an electric shock.

Q.4 What is an earthquake

Marks:2
Ans

An earthquake is a sudden shaking or trembling of the earth. They can cause immense damage to buildings, bridges, dams and people. It results in the severe loss of life and property.

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