Which of the following can make a parallel beam of light when light from a point source is incident on it?
Concave mirror as well as convex lens
Convex mirror as well as concave lens
Two plane mirrors placed at 90° to each other
Concave mirror as well as concave lens
A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is
-30 cm
-20 cm
-40 cm
-60 cm
Under which of the following conditions a concave mirror can form an image larger than the actual object?
When the object is kept at a distance equal to its radius of curvature
When object is kept at a distance less than its focal length
When object is placed between the focus and centre of curvature
When object is kept at a distance greater than its radius of curvature
Figure 10.1 shows a ray of light as it travels from medium A to medium B. Refractive index of the medium B relative to medium A is
\(\dfrac{\sqrt{3}}{\sqrt{2}}\)
\(\dfrac{\sqrt{2}}{\sqrt{3}}\)
\(\dfrac{1}{\sqrt{2}}\)
\(\sqrt{2}\)
A light ray enters from medium A to medium B as shown in Figure 10.2. The refractive index of medium B relative to A will be
greater than unity
less than unity
equal to unity
zero
Beams of light are incident through the holes A and B and emerge out of box through the holes C and D respectively as shown in the Figure 10.3. Which of the following could be inside the box?
A rectangular glass slab
A convex lens
A concave lens
A prism
A beam of light is incident through the holes on side A and emerges out of the holes on the other face of the box as shown in the Figure 10.4. Which of the following could be inside the box?
Concave lens
Rectangular glass slab
Prism
Convex lens
Which of the following statements is true?
A convex lens has 4 dioptre power having a focal length 0.25 m
A convex lens has -4 dioptre power having a focal length 0.25 m
A concave lens has 4 dioptre power having a focal length 0.25 m
A concave lens has -4 dioptre power having a focal length 0.25 m
Magnification produced by a rear view mirror fitted in vehicles
is less than one
is more than one
is equal to one
can be more than or less than one depending upon the position of the object in front of it
Rays from Sun converge at a point 15 cm in front of a concave mirror. Where should an object be placed so that size of its image is equal to the size of the object?
15 cm in front of the mirror
30 cm in front of the mirror
between 15 cm and 30 cm in front of the mirror
more than 30 cm in front of the mirror
A full length image of a distant tall building can definitely be seen by using
a concave mirror
a convex mirror
a plane mirror
both concave as well as plane mirror
In torches, search lights and headlights of vehicles the bulb is placed
between the pole and the focus of the reflector
very near to the focus of the reflector
between the focus and centre of curvature of the reflector
at the centre of curvature of the reflector
The laws of reflection hold good for
plane mirror only
concave mirror only
convex mirror only
all mirrors irrespective of their shape
The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as A, B, C and D in Figure 10.5. Which one of them is correct?
A
B
C
D
You are given water, mustard oil, glycerine and kerosene. In which of these media a ray of light incident obliquely at same angle would bend the most?
Kerosene
Water
Mustard oil
Glycerine
Which of the following ray diagrams is correct for the ray of light incident on a concave mirror as shown in Figure 10.6?
Fig. A
Fig. B
Fig. C
Fig. D
Which of the following ray diagrams is correct for the ray of light incident on a lens shown in Fig. 10.7?
Fig. A
Fig. B
Fig. C
Fig. D
A child is standing in front of a magic mirror. She finds the image of her head bigger, the middle portion of her body of the same size and that of the legs smaller. The following is the order of combinations for the magic mirror from the top.
Plane, convex and concave
Convex, concave and plane
Concave, plane and convex
Convex, plane and concave
In which of the following, the image of an object placed at infinity will be highly diminished and point sized?
Concave mirror only
Convex mirror only
Convex lens only
Concave mirror, convex mirror, concave lens and convex lens
Identify the device used as a spherical mirror or lens in the following cases, when the image formed is virtual and erect in each case.
(i) Object is placed between device and its focus, image formed is enlarged and behind it.
(ii) Object is placed between the focus and device, image formed is enlarged and on the same side as that of the object.
(iii) Object is placed between infinity and device, image formed is diminished and between focus and optical centre on the same side as that of the object.
(iv) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.
(i) Concave mirror
(ii) Convex lens
(iii) Concave lens
(iv) Convex mirror
Why does a light ray incident on a rectangular glass slab immersed in any medium emerge parallel to itself? Explain using a diagram.
The ray emerges parallel to itself because the lateral displacement produced during refraction at the first interface is compensated by the opposite displacement at the second interface. The emergent ray therefore becomes parallel to the incident ray.
A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent if, instead of water, we use liquids like kerosene or turpentine? Support your answer with reason.
No, the pencil will not appear to bend to the same extent. The bending depends on the refractive index of the liquid. Since kerosene and turpentine have refractive indices different from water, the extent of bending will also differ.
How is the refractive index of a medium related to the speed of light? Obtain an expression for the refractive index of a medium with respect to another in terms of speed of light in these two media.
The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in the medium: \( n = \dfrac{c}{v} \).
The refractive index of medium 2 with respect to medium 1 is given by: \( n_{21} = \dfrac{v_1}{v_2} \).
Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.
Given: \( n_{dg} = 1.6 \) and \( n_g = 1.5 \).
The absolute refractive index of diamond is: \( n_d = n_{dg} \times n_g = 1.6 \times 1.5 = 2.40 \).
A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
Yes, the statement is correct.
A magnified virtual image is formed when the object is placed within 20 cm of the lens.
A magnified real image is formed when the object is placed between 20 cm and 40 cm from the lens.
Sudha finds that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?
She should move the screen towards the lens to obtain a sharp image of the building.
The approximate focal length of the lens is 15 cm.
How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm respectively. Which lens will you use to obtain more convergent light?
Power of a lens is inversely proportional to focal length: \( P = \dfrac{1}{f} \).
The 20 cm focal length lens has greater power and will provide more convergent light.
Under what condition in an arrangement of two plane mirrors will the incident ray and reflected ray always be parallel to each other, whatever may be the angle of incidence? Show the same with the help of a diagram.
When two plane mirrors are placed at a right angle (90°) to each other, the incident and reflected rays always emerge parallel regardless of the angle of incidence.
Draw a ray diagram showing the path of rays of light when it enters with oblique incidence (i) from air into water; (ii) from water into air.
When light enters from air to water, it bends towards the normal. When light enters from water to air, it bends away from the normal, as shown in the standard ray diagrams.
Draw ray diagrams showing the image formation by a concave mirror when an object is placed:
(a) Object between pole and focus: The image formed is virtual, erect and enlarged behind the mirror. Rays diverge on reflection but appear to meet behind the mirror.
(b) Object between focus and centre of curvature: The image is real, inverted and enlarged, formed beyond the centre of curvature.
(c) Object at centre of curvature: The image is real, inverted and of the same size, formed at the centre of curvature.
(d) Object beyond centre of curvature: The image is real, inverted and diminished, formed between focus and centre of curvature.
(e) Object at infinity: The image is real, inverted and highly diminished, formed at the focus.
Draw ray diagrams showing the image formation by a convex lens when an object is placed:
(a) Object between optical centre and focus: The image is virtual, erect and enlarged, formed on the same side of the lens as the object.
(b) Object between focus and 2F: The image is real, inverted and enlarged, formed beyond 2F.
(c) Object at 2F: The image is real, inverted and same size, formed at 2F.
(d) Object at infinity: The image is real, inverted and highly diminished, formed at the focus.
(e) Object at focus: The image is formed at infinity, real and inverted.
Write laws of refraction. Explain the same with the help of a ray diagram when a ray of light passes through a rectangular glass slab.
Laws of Refraction:
Explanation with diagram: When a ray of light enters a glass slab from air, it bends towards the normal due to decrease in speed. On emerging from the glass slab into air, it bends away from the normal. However, the emergent ray is parallel to the incident ray because the opposite parallel surfaces cause equal deviation in opposite directions.
Draw ray diagrams showing the image formation by a concave lens when an object is placed:
(a) Object at focus: The image is virtual, erect and diminished, formed between the optical centre and focus on the same side as the object.
(b) Object between F and 2F: The image is virtual, erect and diminished, formed between the optical centre and focus.
(c) Object beyond 2F: The image is virtual, erect and diminished, formed closer to the optical centre.
Draw ray diagrams showing the image formation by a convex mirror when an object is placed:
(a) Object at infinity: The image is virtual, erect and highly diminished, formed at the focus behind the mirror.
(b) Object at finite distance: The image is virtual, erect and diminished, formed behind the mirror between pole and focus.
The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?
Given magnification \( m = 3 \) and image distance \( v = 80 \text{ cm} \)
Using \( m = \dfrac{v}{u} \), we get:
\[ 3 = \dfrac{80}{u} \Rightarrow u = \dfrac{80}{3} = -\,26.7 \text{ cm} \]
The negative sign shows that the object is placed on the same side of the lens as the incident light. The image at 80 cm is real and inverted. Hence, the lens is convex.
The size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to one-third of its size. At what distance has the object been placed from the mirror? What is the nature of the image and the mirror?
Given magnification \( m = \dfrac{1}{3} \) and focal length \( f = -20 \text{ cm} \) (concave mirror).
Using \( m = -\dfrac{v}{u} \) and the mirror formula \( \dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f} \), we get:
\[ -\dfrac{1}{3} = -\dfrac{v}{u} \Rightarrow v = \dfrac{u}{3} \]
Substituting in the mirror equation:
\[ \dfrac{3}{u} + \dfrac{1}{u} = -\dfrac{1}{20} \]
\[ \dfrac{4}{u} = -\dfrac{1}{20} \Rightarrow u = -80 \text{ cm} \]
Thus the object is placed 80 cm from the mirror. The image is real and inverted. The mirror is concave.
Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of −50 cm. What is the nature of the lenses and the power used by each of them?
Power of a lens: It is the reciprocal of focal length in metres: \( P = \dfrac{1}{f} \).
Unit: Dioptre (D).
For focal length \( f = +50 \text{ cm} = 0.5 \text{ m} \):
\[ P = \dfrac{1}{0.5} = 2 \text{ D} \]
The lens is convex.
For \( f = -50 \text{ cm} = -0.5 \text{ m} \):
\[ P = \dfrac{1}{-0.5} = -2 \text{ D} \]
The lens is concave.
A student focused the image of a candle flame on a white screen using a convex lens. He noted down the positions of the candle, lens and screen. Using the recorded positions, answer the following:
Given:
Position of candle = 12 cm
Position of lens = 50 cm
Position of screen = 88 cm
(i) Focal length:
Object distance \( u = 12 - 50 = -38 \text{ cm} \)
Image distance \( v = 88 - 50 = 38 \text{ cm} \)
Using lens formula:
\[ \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f} \]
\[ \dfrac{1}{38} - \left( -\dfrac{1}{38} \right ) = \dfrac{2}{38} = \dfrac{1}{19} \]
Thus, \( f = 19 \text{ cm} \).
(ii) If the candle is shifted to 31 cm: The image forms at infinity.
(iii) If the candle is moved still farther: The image becomes virtual and erect.
(iv) Ray diagram: (Diagram shows a convex lens with object between focus and optical centre, rays diverging but appearing to meet behind lens forming virtual image.)