Class 10 › Science Light – Reflection and Refraction Class 10 Science · Chapter 9
Practice all → Summary Questions & answers Practise this chapter → View all Page 1 free about 8 min read , Full kit about 55 min read Page 1 free about 8 min read , Full kit about 55 min read View all 12 amazing Marks Booster kits → Light travels in straight lines and changes direction when it meets surfaces or different media. Reflection follows two laws — the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in one plane. Spherical mirrors (concave and convex) form images whose nature, position and size are found using ray diagrams and the mirror formula 1/v + 1/u = 1/f, with magnification m = −v/u. Refraction is the bending of light as it passes from one medium to another, governed by Snell’s law and described by the refractive index. Lenses (convex and concave) form images using the lens formula 1/v − 1/u = 1/f, and the power of a lens is measured in dioptres. These ideas explain everyday optics from mirrors and magnifiers to cameras.
Laws of reflection
Angle of incidence equals angle of reflection; rays and normal are coplanar.
Refractive index
A measure of how much a medium bends light.
Mirror/lens formula
Relation linking object distance, image distance and focal length.
Power of a lens
The reciprocal of focal length (in metres), measured in dioptres.
Q1 Why does a ray of light bend when it enters water from air? Answer Q2 What kind of mirror is used as a rear-view mirror in vehicles, and why? Answer Q3 State the laws of reflection of light. Answer Q4 Where should an object be placed before a concave mirror to get a magnified, virtual image? Answer Q5 Why is a convex mirror used as a rear-view mirror in vehicles? Answer Q6 Write the mirror formula and the magnification formula. Answer Q7 What is refraction of light, and why does it occur? Answer Q8 What is the refractive index of a medium? Answer Q9 Write the lens formula and explain the power of a lens. Answer Q10 How does a convex lens form a real, inverted image, and where is it used? Answer Q1 1. Define the principal focus of a concave mirror. Answer Q2 2. The radius of curvature of a spherical mirror is \(20\ \text{cm}\). What is its focal length? Answer Q3 3. Name a mirror that can give an erect and enlarged image of an object. Answer Q4 4. Why do we prefer a convex mirror as a rear-view mirror in vehicles? Answer Q1 1. Find the focal length of a convex mirror whose radius of curvature is \(32\ \text{cm}\). Answer Q2 2. A concave mirror produces three times magnified (enlarged) real image of an object placed at \(10\ \text{cm}\) in front of it. Where is the image located? Answer Q1 1. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why? Answer Q2 2. Light enters from air to glass having refractive index \(1.50\). What is the speed of light in the glass? The speed of light in vacuum is \(3 \times 10^8\ \text{m s}^{-1}\). Answer Q3 3. Find out, from Table 9.3, the medium having highest optical density. Also find the medium with lowest optical density. Answer Q4 4. You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 9.3. Answer Q5 5. The refractive index of diamond is \(2.42\). What is the meaning of this statement? Answer Q1 1. Define \(1\) dioptre of power of a lens. Answer Q2 2. A convex lens forms a real and inverted image of a needle at a distance of \(50\ \text{cm}\) from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens. Answer Q3 3. Find the power of a concave lens of focal length \(2\ \text{m}\). Answer Q1 1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay Answer Q2 2. The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus. Answer Q3 3. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus. Answer Q4 4. A spherical mirror and a thin spherical lens have each a focal length of \(-15\ \text{cm}\). The mirror and the lens are likely to be
(a) both concave.
(b) both convex.
(c) the mirror is concave and the lens is convex.
(d) the mirror is convex, but the lens is concave. Answer Q5 5. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
(a) only plane.
(b) only concave.
(c) only convex.
(d) either plane or convex. Answer Q6 6. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length \(50\ \text{cm}\).
(b) A concave lens of focal length \(50\ \text{cm}\).
(c) A convex lens of focal length \(5\ \text{cm}\).
(d) A concave lens of focal length \(5\ \text{cm}\). Answer Q7 7. We wish to obtain an erect image of an object, using a concave mirror of focal length \(15\ \text{cm}\). What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case. Answer Q8 8. Name the type of mirror used in the following situations.
(a) Headlights of a car.
(b) Side/rear-view mirror of a vehicle.
(c) Solar furnace.
Support your answer with reason. Answer Q9 9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations. Answer Q10 10. An object \(5\ \text{cm}\) in length is held \(25\ \text{cm}\) away from a converging lens of focal length \(10\ \text{cm}\). Draw the ray diagram and find the position, size and the nature of the image formed. Answer Q11 11. A concave lens of focal length \(15\ \text{cm}\) forms an image \(10\ \text{cm}\) from the lens. How far is the object placed from the lens? Draw the ray diagram. Answer Q12 12. An object is placed at a distance of \(10\ \text{cm}\) from a convex mirror of focal length \(15\ \text{cm}\). Find the position and nature of the image. Answer Q13 13. The magnification produced by a plane mirror is \(+1\). What does this mean? Answer Q14 14. An object \(5.0\ \text{cm}\) in length is placed at a distance of \(20\ \text{cm}\) in front of a convex mirror of radius of curvature \(30\ \text{cm}\). Find the position of the image, its nature and size. Answer Q15 15. An object of size \(7.0\ \text{cm}\) is placed at \(27\ \text{cm}\) in front of a concave mirror of focal length \(18\ \text{cm}\). At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image. Answer Q16 16. Find the focal length of a lens of power \(-2.0\ \text{D}\). What type of lens is this? Answer Q17 17. A doctor has prescribed a corrective lens of power \(+1.5\ \text{D}\). Find the focal length of the lens. Is the prescribed lens diverging or converging? Answer
According to the laws of reflection, the angle of incidence is:
A greater than the angle of reflection B less than the angle of reflection C always 90° D equal to the angle of reflection
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