CBSE Class 10 Science Light Reflection and Refraction Assignment Set G

Very Short Answer Type Questions

Question. Give the relation between focal length and radius of curvature.
Answer: f = R/2

Question. Where should be an object placed in front of convex lens so as to use it as a magnifier?
Answer: Between the pole and the focal length.

Question. Define the term angle of incidence. 
Answer: The angle between an incident ray and the normal at the point of incidence is called angle of incidence.

Question. What is a ray of light? 
Answer: The path along which light travels is called a ray of light.

Question. The power of a lens is –4.0 D. What is the nature of this lens?
Answer: Negative power is associated with only concave lens.

Question. Name some phenomenon associated with light during image formation by mirrors.
Answer: Reflection.

Question. Define reflection of light.
Answer: The phenomenon of coming back of light in the same medium after striking a plane and polished surface is called reflection of light.

Question. State laws of reflection.
Answer: Incident ray, reflected ray and normal at the point of incidence all lie in the same plane. The angle of incidence is equal to the angle of reflection.

Question. What is the name given to the centre of the mirror ? 
Answer: The centre of reflecting surface of a spherical mirror is known as Pole.

Question. What is an optically denser medium?
Answer: A medium in which light travels comparatively slower than the other medium is called an optically denser medium.

Question. What is silvering of mirror?
Answer: Silvering of mirror means coating the surface of mirror with a thin layer of silver, aluminium or some other shiny, opaque material.

Question. What are the two types of lenses?
Answer: Spherical lens: combination of two spherical refracting surfaces. 

Short Answer Type Questions

 Question. Define refractive index.
Answer: Refractive index is defined as the ratio of speed of light in medium 1 to the speed of light in medium 2 and is represented as n21 and is read as refractive index of medium 2 with respect to medium 1.
n21 = speed of light in medium 1/speed of light in medium 2.

Question. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 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: Focal length of concave mirror = 15 cm.
Object should be place in front of given concave mirror at a distance less than 15 cm.
Image formed is virtual and erect.
Image size is larger than object.

Question. Name the type of mirror used in the following situations.
(a) Headlight of a car.
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace.
Support your answer with reason.
Answer: (a) For headlight of a car— Concave mirror is used to get a powerful beam of light after reflection.

(b) Convex mirror is used for side/rear view mirror of a vehicle. Convex mirror forms an erect and diminished image of vehicles and gives wider view of rear.
(c) In solar furnace concave mirror is used as a reflector, it concentrates sun light at a point where the temperature increases sharply to 180°C – 200°C.

Question. 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: Yes, one-half of a convex lens when covered with a black paper, the lens produces a complete or full image of an object.
To verify experimentally:
Take a convex lens, cover half part of it as shown in the figure, with a paper. Place it on a stand. Focus a distant object on a screen, the image obtained on the screen is complete.
Observation and conclusion: Image formed on the screen does not depend on the size of the lens. The brightness of the image decreases as less number of rays pass through the lens.   (Img 145)

Question. Define pole, centre of curvature, radius of curvature, principal axis, aperture, focus and focal length of a spherical mirror.
Answer: Pole: the centre of reflecting surface. It is represented by letter P.
Centre of Curvature: The centre of the sphere of which the mirror forms the part. Represented by “C”.

Question. What is light?
Answer: Light is a form of electromagnetic radiation that causes the sensation of sight. It doesn’t require any material medium to travel.
Radius of Curvature: The radius of the sphere of which the mirror forms the part.
Represented by “R”.
Principal axis: The straight line joining the pole (P) and the centre of curvature. It is normal to the mirror at its pole.
Aperture: The diameter of the spherical mirror is called its aperture. The reflecting surface of the mirror.
Focus: The point of the principal axis at which the rays parallel to principal axis meet (concave mirror) or appear to meet (convex mirror) after reflection. Represented by F.
Focal Length: The distance between the pole and the principal focus of a spherical mirror is called focal length. Represented by f.

Question. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Answer: Concave lens 
f = –15 cm
ν = –10 cm
1/v – 1/u = 1/f
1/-10 – 1/u = 1/-15
∴ 1/u = – 1/30 ∴ u = –30 cm

Question. Define pole and centre of curvature of spherical mirrors.
Answer: Pole: The centre of the reflecting surface of a spherical mirror is a point called the pole. It lies on the surface of the mirror. It is represented by “P”. Centre of Curvature: The reflecting surface of a spherical mirror forms a part of a sphere this sphere has a centre and this point is called the centre of curvature of the spherical mirror.

Question. If the speed of light in water is 2.25 × 108 m/s and the speed in vacuum is 3×108m/s.
Calculate the refractive index of water.
Answer: Refractive index of water =  Speed of light in 1 medium (air) / Speed of light in 2 medium (water)
nm = c/v
nm = 3 x 108 / 2.25 x 108
nm = 1.33
∴The refractive index of water = 1.33.

Question. The refractive index of water is 1.33 and kerosene is 1.44. Calculate the refractive index of kerosene with respect to water.
Answer: Refractive index of water = n_w = 1.33
Refractive index of kerosene = n_k = 1.44
∴Refractive index of kerosene with respect to water is
n_kw =  = 1.44/1.33   
= 1.082

Question. Redraw the given diagram and show the path of refracted ray.   

Answer: 

Question. A convex lens has a focal length of 10 cm. At what distance from the lens should the object be placed so that it gives a real and inverted image 20 cm away from the lens?
What would be the size of the image formed if the object is 2 cm high? With the help of a ray diagram show the formation of the image by the lens in this case. 
Answer: f = +10 cm, ν = +20 cm as image is real and inverted. Height of the object = 2 cm (say +ve)
Using 1/f = 1/v – 1/u , we get
1/u = 1/v – 1/f
= 1/+20 – 1/10 = +1-2 / 20 = -1/20
Image will be of the same size as that of object (as u = ν) and hence, the height of the image will be 2 cm. 

Question. Redraw the given diagram and show the path of the refracted ray. 

Answer: 

Long Answer Type Questions

Question. Draw ray diagrams to show the image formed by a concave lens for the object placed at (i) infinity (ii) Between f and 2f of the lens.
Answer: (i) object at infinity 

Object ⎯→ between f and 2f
image ⎯→ between O and F.
Size of image ⎯→ Diminished
Nature ⎯→ virtual and erect.

Question. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.
Answer: 

Hence, the image formed at 16.67 cm from the lens on the other side. The size of the image is 3.3 cm, i.e., reduced and inverted.

Question. A concave lens has a focal length of 10 cm. An object 2.5 cm high is placed 30 cm from the lens. Determine the position and size of the image.
Answer: Since the lens is concave, hence f is negative
Given: u = – 30 cm; f= – 10 cm; h = 2.5 cm; v = ?; h’ = ?

Questions On High Order Thinking Skills

Question. Write the laws of reflection.
Answer: (a) The angle of incidence is equal to angle of reflection.
(b) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Question. Give characteristics of image formed by plane mirror
Answer: Image is virtual and erect.
• Size is same as of the object.
• It is formed at same distance.

Question. Give uses of plane mirror.
Answer: • Looking glass
• Used in submarines
• Solar cooker
• Kaleidoscope

Question. Name two types of spherical mirror.
Answer: • Concave mirror
• Convex mirror

Question. Give uses of concave mirror.
Answer: • Used as reflectors in car headlights, searchlights, etc.
• Used as shaving mirror.
• Used in solar cooker to focus the sunlight on one point.

Question. Give uses of convex mirror.
Answer: • It is used as the rear view mirror in cars.
• It is used in street lights as it diverge the light over larger area.

Question. What are the two types of refractive index?
Answer: • Relative refractive index– It is the ratio of speed of light in one medium to the speed of light in another medium
• Absolute refractive index– It is the ratio of light in vacuum to the speed of light in another medium

Question. Why do we prefer a convex mirror as a rear view mirror in vehicles?
Answer: Convex mirrors are used as rear view mirror in cars because it produces erect and diminished image of the traffic behind the vehicle. It also gives a wider view.

Question. 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.
Answer: (a) Concave mirror
(b) Convex mirror
(c) Concave mirror