PHYSICAL SCIENCE LESSON-4 REFRACTION OF LIGHT AT CURVED SURFACES

PHYSICAL SCIENCE

LESSON-4

REFRACTION OF LIGHT AT CURVED SURFACES

 Reflections on concepts

Question 1.
How do you verify experimentally that the focal length of a convex lens is increased when it is kept in water?
Answer:
Aim: To prove that the focal length of a convex lens is increased when it is kept in water.
Apparatus: Convex lens of known focal length, circular lens holder, tall cylindrical glass tumbler, black stone, water.

Procedure:

• Take a cylindrical glass tumbler whose height is much greater than the focal length of the lens and fill it with water.
• Keep a black stone at the bottom of the vessel.
• Now dip the lens Into water using circular lens holder such that it is at a distance which Is less than or equal to focal length of the lens in air.
• Now see through the lens to have a view of the black stone.
• Now increase the height of the lens till you are not able to see the stone’s image.
• When the lens is dipped to a height which is greater than the focal length of lens in air, we are able to see the image. Showing that focal length of the lens has Increased in water.
• From this we conclude that the focal length of a convex lens is increased when it Is kept In water.

Question 2.
How do you find the focal length of a lens experimentally?
Answer:

• Take a v-stand and place it on a long table at the middle.
• Place a convex lens on the v-stand. Imagine the principal axis of the lens.
• Light a candle and ask your friend to take the candle far away from the lens along the principal axis.
• Adjust a screen (a sheet of white paper placed perpendicular to the axis) which is on other side of the lens until you get an image on it.
• Measure the distance of the image from the v-stand of lens (image distance v) and also measure the distance between the candie and stand of lens (object distance ‘u’). Record the values in the table.

Object Distance ‘u’	Image Distance ‘v’	Focal length ‘f’

• Now place the candle at a distance of 60 cm from the lens, try to get an image of the candle flame on the other side on a screen. Adjust the screen till You get a clear image.
• Measure the image distance ‘y’ and object distance ‘u’ and record the values in table.
• Repeat the experiment for various object distances like 50 cm, 40cm, 30cm etc. Measure the image distances in all cases and note them in table.
• Using the formula 1f=1v−1u find f in all the cases. We will observe the value f’ is equal in all cases. This value of ‘f is the focal length of the given lens.

 

Question 3.
Draw ray diagrams for the following positions and explain the nature and position of image.
(i) Object is placed at C₂
(ii) Object is placed between F₂ and optic centre P.
Answer:

i) 

When an object is placed at the centre of curvature (C2) on the principal axis, we will get an Image at (C1) which is real, inverted and of the same size as that of the object

If the object is placed between focus and optic centre, we will get an image which is virtual, erect and magnified.

Question 4.
Write the lens maker’s formula and explain the terms in it.
Answer:
Lens maker’s formula is = 1/f=(n−1)(1/R1−1/R2)
f = focal length of the lens
n = refractive index of the lens
R, and R₂ are the radii of curvatures of two surfaces of the lens.

Application On Concepts

Question 1.
Two converging lenses are to be placed in the path of parallel rays so that the rays remain parallel after passing through both lenses. How should the lenses be arranged? Explain with a neat ray diagram.
Answer:

• A parallel beam of light rays will converge on focal point of the lens.
• light rays passing through focal point will emerge parallel to principal axis, the two lenses should be arranged as shown.
• The two lenses are arranged on a common principal axis such that their focal point coincides with each other.

Question 2.
The focal length of a converging lens is 20cm. An object Is 60cm from the lens. Where will the image be formed and what kind of ¡mage is It?
Answer:
f = 20 cm (convex lens, f = + ve)
u = – 60 cm [object distance = – ve]
v = ?
Lens formula : 1f=1v−1u⇒120=160+1v=120−160⇒1v=3−160=260=130
∴ v= 30 cm.
∴ The image distance is 30 cm.
f = 20 cm; hence, R = 40 cm, Object distance = 60 cm
∴ The object is placed beyond centre of curvature. Hence the image formed is real. inverted and diminished at 30 cm from the Lens.

 

Question 3.
A double convex lens has two surfaces of equal radii ‘R’ and refractive index n = 1.5. Find the focal length ‘f.
Answer:
n = 1.5
R₁ = R₂ = R
Lens maker’s formula for convex lens : 1/f=(n−1)(1/R1−1/R2)
= 1f=(1.5−1)(1R+1R)
= 0.5 × 2R=1R
∴ f = R

Question 4.
Find the refractive index of the glass which is a symmetrical convergent lens it its local length is equal to the radius of curvature of its surface.
Answer:
Given that lens is convergent symmetrical We know that
∴ R₁ = R = f
R₂ = -R= -f
We know that 1/f=(n−1)[1/R1−1/R2]

∴ Refractive index of glass = 1.5

Question 5.
Man wants to get a picture of a zebra. He photographed a white donkey after fitting a glass with black stripes, onto the lens of his camera. What photo will he get? Explain.
Answer:

1. He will get a photograph which consists of black and white stripes.
2. As the reflected light rays from the white donkey entered into camera through the lens having black stripes, these black stripes do not allow the rays Inside.
3. So, the rays which pass through the transparent part of a camera lens only forms the corresponding image of donkey on the film i.e., white lines as it is white in colour.

Question 6.
Harsha tells Slddhu that the double convex lens behaves ‘like a convergent lens. But Slddhu knows that Harsha’s assertion Is wrong and corrected Harsha by asking some questions. What are the questions asked by Siddhu?
Answer:
Siddhu may ask the following questions

1. What is the shape of the lens If two convex null lenses are attached?
2. What happens when a light ray passes through a double convex lens?
3. Is there any convergent point available, if the light ray passes through a double convex lens.

Question 7.
Can a virtual Image be photographed by a camera?
Answer:

• Yes, a virtual Image can be photographed by a camera.
• A plane mirror forms a virtual Image, we can able to take photographs of that Image In plane mirror.
• In the same way, human eyes forms a virtual image, which can able to take a photograph.

Question 8.
How do you appreciate the coincidence of the experimental facts with the results obtained by a ray diagram in terms of behaviour of Images formed by lenses?
Answer:

1. By using a ray diagram, the reflected ray must be placed at a particular point by the principal axis. That means we have to find the images, shorter or longer.
2. By using ray diagrams, we are able to find the focal length from lens maker’s formula in many optical instruments, some lens combinations are used to magnification (or) diminished of the image.
3. When white light passes through a prism, then VIBGYOR is formed on the principal axis that means converging take place.
4. So, I appreciate the coincidence of the experimental facts with the results obtained by a ray diagram in terms of behaviour of images formed by lenses.

Question 9.
Find the radii of curvature of a convexo-concave convergent lens made of glass with refractive Index n = 1.5 having focal length of 24cm. One of the radii of curvature is double the other.
Answer:
n=1.5, f=24cm, R₁=R, R₂=2R
1/f=(n−1)(1/R1+1/R2)
R₁ = positive, R₂ = positive
124=(1.5−1)(1/R−1/2R)
124=(0.5)(2−1/2R)
124=(0.5)(1/2R)
R = 6 cm

R1 = R = 6cm,
R2 = 2R = 12 cm

 

Multiple choices questions

Question 1.
Which one of the following materials cannot be used to make a lens? (   )
(A) water
(B) glass
(C) plastic
(D) clay
Answer:
(D) clay

Question 2.
Which of the following is true? (   )
(A) The distance of virtual image is always greater than the object distance for convex lens.
(B) The distance of virtual image is not greater than the object distance for convex lens.
(C) Convex lens always forms a real image.
(D) Convex lens always forms a virtual image.
Answer:
(B) The distance of virtual image is not greater than the object distance for convex lens.

Question 3.
Focal length of the piano-convex lens is ……………………… when its radius of curvature of the surface is R and n is the refractive index of the lens. ( )
(A) f = R
(B) f=R/2
(C) f=R/(n-1)
(D) f=(n-1)/R
Answer:
(C) f=R/(n-1)

Read More: ACID BASES AND SALT