CBSE Important Questions Class 10 Science Chapter 9 – Light Reflection and Refraction

Class 10 Science Chapter 9 Light – Reflection and Refraction is part of the CBSE Class 10 Science syllabus. It explains how light travels in a straight line, how it is reflected by plane, concave and convex mirrors, how images are formed at different positions of the object, and how light bends (refracts) when it passes from one medium to another. Students also learn the mirror formula, the lens formula, magnification and the sign conventions used in numerical problems.

These CBSE important questions for Class 10 Science Chapter 9 cover the image formed by a concave lens, why a plane mirror image cannot be caught on a screen, the focus and focal length of a convex mirror, ray diagrams for a concave mirror, and numerical problems on magnification and the mirror formula. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. An object is placed in front of a concave lens. Describe the nature, orientation and relative size of the image formed.

Answer: A concave lens always forms the same kind of image, wherever the object is placed.
1. Nature: the image is virtual, so it cannot be caught on a screen.
2. Orientation: the image is erect (upright).
3. Size: the image is diminished, that is, smaller than the object.
The image is always formed between the focus and the optical centre, on the same side as the object.

Q2. A student claims that the image formed by a plane mirror can be obtained on a screen. Is this statement correct? Justify your answer by explaining the nature of the image formed.

Answer: The statement is incorrect. A plane mirror forms a virtual image.
In a virtual image the reflected rays only appear to come from a point behind the mirror; they do not actually meet there. Since no light actually reaches that place, nothing can be caught on a screen held there.
Only a real image, formed when light rays actually meet, can be obtained on a screen.

Q3. Read the statements below and choose the correct option.
Assertion (A): The principal focus of a convex mirror is situated in front of the mirror.
Reason (R): The ray of light diverges after reflection from the convex mirror.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Answer: (d) A is false, but R is true.
A convex mirror spreads out (diverges) the reflected rays, so R is true. Because the rays diverge, they only appear to come from a point behind the mirror. So the principal focus of a convex mirror lies behind the mirror, not in front of it, which makes A false.

Q4. The nature of the image formed by a plane mirror is
(a) real and diminished
(b) virtual and enlarged
(c) virtual and of the same size as that of the object
(d) real and same size as that of object

Answer: (c) virtual and of the same size as that of the object
A plane mirror always forms a virtual, erect image that is exactly the same size as the object, and it is formed as far behind the mirror as the object is in front of it.

Q5. The focal length of a convex mirror is ________.
(a) positive
(b) negative
(c) depends on the nature of the image
(d) depends on the position of the object

Answer: (a) positive
In the Cartesian sign convention, distances measured behind the mirror are taken as positive. The focus of a convex mirror lies behind the mirror, so its focal length is positive. For a concave mirror the focus is in front, so its focal length is negative.

Q6. A person wants to obtain the image of a sharp needle on a screen 36 cm in front of a mirror by keeping the needle at a distance of 6 cm from its pole.
A) How far is the image from the object?
B) What will be the linear magnification of the image produced?

Answer: Since the image is obtained on a screen, it is a real image formed in front of the mirror.
So, image distance v = −36 cm and object distance u = −6 cm.
A) Distance between image and object = |v − u| = |−36 − (−6)| = |−30| = 30 cm.
B) Magnification m = −v/u = −(−36)/(−6) = −6
The negative sign shows the image is real and inverted, and it is 6 times larger than the object.

Q7. An object is placed at a distance of 20 cm from a convex mirror. What will be the magnification produced if the image is formed 10 cm behind the mirror?

Answer: Here, v = +10 cm (behind the mirror) and u = −20 cm.
m = −v/u = −(+10)/(−20) = +0.5
The positive sign shows the image is virtual and erect, and since the value is less than 1, the image is diminished. This is exactly what a convex mirror always produces.

Q8. The radius of curvature of a mirror is +30 cm. Find the nature and focal length of the mirror.

Answer: Focal length is half the radius of curvature:
f = R/2 = +30/2 = +15 cm
The focal length is positive, which means the focus lies behind the mirror. So the mirror is a convex (diverging) mirror with a focal length of 15 cm.

Q9. An object is placed at the centre of curvature of a concave mirror. Identify the option that correctly explains the property of the image formed.
(a) An upright image of the same size as the object forms at the focus
(b) An inverted image of the same size as the object forms at the centre of curvature
(c) An upright and diminished image forms beyond the centre of curvature
(d) An inverted diminished image forms between the centre of curvature and focus

Answer: (b) An inverted image of the same size as the object forms at the centre of curvature.
When the object is at C, the reflected rays meet again at C. The image formed there is real, inverted and exactly the same size as the object. This is the only position where a concave mirror makes an image of the same size as the object.

Q10. An object is placed in front of a concave mirror as shown. Identify the correct statement for the image formed.
(a) An inverted and diminished image will form beyond the centre of curvature.
(b) An upright image of the same size will form at the centre of curvature.
(c) An inverted and diminished image will form between the centre of curvature and focus.
(d) An upright image of the same size will form between pole and the focus.

Answer: (c) An inverted and diminished image will form between the centre of curvature and focus.
In the figure the object is placed beyond C. For such a position a concave mirror always forms an image that is real, inverted and smaller than the object, and it lies between C and F.

Q11. An object of height 5 cm is kept at a distance of 87 cm from the pole of a diverging mirror. If the focal length of the mirror is 29 cm, the height of the image formed is
(a) 1.25 cm
(b) 2.5 cm
(c) 5 cm
(d) 3.75 cm

Answer: (a) 1.25 cm
A diverging mirror is a convex mirror, so f = +29 cm and u = −87 cm.
Using the mirror formula 1/v + 1/u = 1/f:
1/v = 1/29 + 1/87 = 3/87 + 1/87 = 4/87
v = 87/4 = 21.75 cm
m = −v/u = −21.75/(−87) = +0.25
Height of image = m × height of object = 0.25 × 5 = 1.25 cm

Q12. Explain why it is not possible to view the flame of a lighted candle through a bent pipe.

Answer: Light travels in a straight line. This property is called the rectilinear propagation of light.
In a bent pipe the path is not straight, so the light coming from the candle flame cannot bend round the curve to reach the eye. Since no light from the flame reaches the eye, the flame cannot be seen. If the pipe is made straight, the flame becomes visible at once.

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