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Page No 211:

Question 1:

Which diagram correctly shows the path of a ray of light after refraction through a convex lens?
Figure

Answer:

The diagram 'd' correctly shows the path of light after refraction through a convex lens.
A ray of light passing through the focus of a convex lens becomes parallel to its principal axis after refraction through the lens.

Page No 211:

Question 2:

The hole in the eye that controls the amount of light that enters is the
(a) iris.
(b) pupil.
(c) cornea.
(d) sclera.

Answer:

(b) pupil
The pupil is a hole in the middle of the iris; it controls the amount of light entering the eye.

Page No 211:

Question 3:

The cornea is a transparent structure in the eye which
(a) refracts light.
(b) reflects light.
(c) absorbs light.
(d) focuses light.

Answer:

(d) focuses light
The outer surface of the cornea is convex; here, the focusing of light begins.

Page No 211:

Question 4:

When an object moves towards a convex lens from beyond 2F upto F, the size of the image _____________.

(a) decreases
(b) increases
(c) remains the same
(d) first decreases then increases

Answer:

(b) increases
When an object is moved beyond 2F up to the focus of a convex lens, the size of the image increases. At focus, the image is highly enlarged.

Page No 211:

Question 5:

When an object moves closer to a convex lens, always remaining beyond F, the image formed by it shifts _____________.

(a) away from the lens
(b) first away and then towards the lens
(c) towards the lens
(d) first towards and then away from the lens

Answer:

(a) away from the lens
When an object moves closer to a convex lens, always remaining beyond F, the image formed by it shifts away from the lens.

Page No 211:

Question 6:

The arrow in the diagram represents a beam of light striking a triangular glass prism. On entering the glass, the beam
Figure

(a) bends towards A.
(b) bends towards B.
(c) is totally reflected back.
(d) continues without bending.

Answer:

(b) bends towards B
The beam of light bends as it goes from one transparent medium (air) to another medium (glass). Because air is a rarer medium and glass is a denser medium, the beam is refracted towards the normal; thus, it bends towards B.

Page No 211:

Question 7:

The arrow in the figure represents a ray of light moving from air into the surface of water at Q. After entering water, the ray
Figure

(a) will be completely reflected without change in speed.
(b) will be refracted with increased speed.
(c) will be refracted with reduced speed.
(d) will be refracted without change in speed.

Answer:

(c) will be refracted with reduced speed
When a ray of light moves from air to water, its speed gets reduced; this change in the speed of light causes the refraction of light.



Page No 212:

Question 8:

A virtual image formed by a convex lens is
(a) always inverted.
(b) always smaller than the object.
(c) formed on the screen sometimes.
(d) always upright.

Answer:

(d) always upright
A virtual image formed by a convex lens is always erect/upright and enlarged; it cannot be obtained on a screen.

Page No 212:

Question 9:

The images formed by converging lenses
(a) can be real or virtual depending on the position of the object.
(b) are always virtual and magnified.
(c) are always real and inverted.
(d) are always real.

Answer:

(a) can be real or virtual depending on the position of the object
The images formed by converging lenses can be real or virtual depending on the position of the object.

Page No 212:

Question 10:

Converging lenses produce images that
(a) are virtual and inverted.
(b) are real and inverted.
(c) may be real or virtual, upright or inverted.
(d) are real and upright.

Answer:

(c) may be real or virtual and upright or inverted
Converging lenses produce images that are real or virtual and upright or inverted, depending upon the position of the object.

Page No 212:

Question 11:

When an object is within the focal length of a convex lens, the image formed is
(a) real and inverted.
(b) virtual and magnified.
(c) virtual and the same size.
(d) real and magnified.

Answer:

(b) virtual and magnified
When an object is placed within the focal length of a convex lens, i.e., between the focus and the lens, a virtual, erect and magnified image is formed.

Page No 212:

Question 12:

Look carefully at the picture. Now, choose and write down the correct statements.
Figure

(a) The pen bends at the water line.
(b) The pen appears to bend at the water line.
(c) The pen is not really bent.
(d) The light rays are bent.
(e) The light rays make the pen look as if it is bent.
(f) When light rays bend, it is called refraction.

Answer:

The correct statements are:
(b) The pen appears to be bent at the water line.
(c) The pen is not really bent.
(d) The light rays are bent.
(e) The light rays make the pen look as if it is bent.
(f) When light rays bend, it is called refraction.

Page No 212:

Question 13:

Indicate whether the following statements are true or false.
(a) Two plane mirrors placed at an angle of 60° will produce five images.
(b) As the angle between the mirrors decreases, the number of images formed also decreases.
(c) The turning of a ray of light as it passes from one medium to another is called refraction.
(d) When a ray of light passes from glass to air, the refracted ray moves towards the normal.
(e) A concave lens is a diverging lens.
(f) A convex lens is a diverging lens as rays parallel to the principal axis meet at a point after refraction.
(g) The bottom of a swimming pool seems higher than it actually is because of refraction of light.
(h) A convex lens produces only real images.
(i) The cornea gives colour to the eye.
(j) Vitamin A is essential for good eyesight.
(k) A magnifying glass forms a virtual, inverted image.
(l) Concave lenses bulge outwards, towards the incident light.
(m) A ray of light passing through the focus of a lens is parallel to the axis after refraction.
(n) Long-sighted people always need to wear spectacles.

Answer:

(a) True

(b) False
As the angle between the mirrors decreases, the number of images formed increases.

[Numbers of images formed, n = (360/Angle between two mirrors) - 1]

(c) True

(d) False
When a ray of light passes from glass to air, the refracted ray moves away from the normal.

(e) True

(f) False
A convex lens is a converging lens, as rays parallel to the principal axis meet at a point after refraction.

(g) True

(h) False
A convex lens may form either a real image or a virtual image depending upon the position of the object.

(i) False
The iris is the coloured part of the eye and the cones allow the eye to see in colour and in bright light.

(j) True

(k) False
A magnifying glass forms a virtual, erect image.

(l) False
Convex lenses bulge outwards towards the incident light.

(m) True

(n) True



Page No 213:

Question 14:

Find the errors in the diagrams given here.
Figure

Answer:

(a) Error - The bending of light is not shown.
When a light ray passes from one medium to another, it bends/refracts from its path.

(b) Error - The deviation of a light ray when it goes from glass to air is not shown correctly and the direction of emergent ray is shown parallel to the original direction of incident ray. It is not possible in case of a prism.

(c) Error - The bending of light is shown.
If the incident ray falls normally on the surface of a glass slab, then there is no bending of the ray; it goes straight.

(d) Error - The light ray is shown parallel to its principal axis after refraction.
A light ray passing through the optical centre of a convex lens goes straight after refraction through the lens.

(e) Error - The ray is shown straight after refraction.
A light ray passing through the focus of a convex lens becomes parallel to its principal axis after refraction.

(f) Error- The ray is passing through the centre of curvature after refraction.
A light ray parallel to the principal axis passes through its focus after refraction through the lens.

(g) Error - The concave lens is diverging a parallel beam of light.
A concave lens always converges the parallel beam of light.

Page No 213:

Question 15:

Copy the diagrams shown and complete the path of each light ray as it passes through the lens.
Figure

Answer:

(i)

(ii)



(iii)


(iv)

Page No 213:

Question 16:

A man 1 m tall steps into a stream which looks 90 cm deep. He finds himself completely submerged in water from head to toe. He was tricked! How did that happen?

Answer:

It happened because of the refraction of light rays that pass from the stream to air. When a man looks into a stream, it appears to be less deep than it really is. Therefore, when a 1 m (100 cm) tall man steps into a stream that looks 90 cm deep, he finds himself completely submerged.

Page No 213:

Question 17:

Read the paragraph given below carefully. Underline the mistakes in it and then rewrite the paragraph correctly in your notebook.

Long sight or myopia is a defect in the eye that is more common in old age. People suffering from this defect can read a book clearly, but find it difficult to see a distant road sign. The image of the object, instead of forming in front of the retina, forms behind it. This defect can be corrected by using a convex or a diverging lens.

Answer:

Long sight or myopia is a defect in the eye that is more common in old age. People suffering from this defect can read a book clearly, but find it difficult to see a distant road sign. The image of the object, instead of forming in front of the retina, forms behind it. This defect can be corrected by using a convex or a diverging lens.

The paragraph should be rewritten as follows:
Long-sightedness or hypermetropia is a defect in the eye that is more common in old age. People suffering from this defect cannot read a book clearly, but find it easy to see a distant road sign. The image of the object, instead of forming on the retina, forms behind it. This defect can be corrected by using a convex or converging lens.

Page No 213:

Question 18:

In the picture of the eye shown here, name the parts indicated by arrows.
Figure

Answer:

Page No 213:

Question 19:

Draw ray diagrams to show how a ray of light passes through the following:
(a) a parallel sided glass block.
(b) a glass prism

Answer:

(a) A parallel-sided glass block

Here,

i = Angle of incidence

r = Angle of refraction

r' = Angle of incidence at the emergent point

(b) A glass prism


 

Here,

Line PQ = Incident ray

Line QR = Refracted ray

Line RS = Emergent ray

Angle i = Angle of incidence

Angle r1 = Angle of refraction

Angle e = Angle of emergence

Angle  = Angle of deviation

Page No 213:

Question 20:

Give a word or phrase for the following:
(a) the point where rays of light parallel to the principal axis converge after refraction in a convex lens
(b) the hole which lets light into the eye
(c) the ability to see only objects nearby clearly
(d) the part of the eye where the image is formed
(e) an image which cannot be obtained on a screen
(f) an instrument used to look at details of tiny objects

Answer:

(a) Principal focus
(b) Pupil
(c) Short-sightedness
(d) Retina
(e) Virtual image
(f) Microscope

Page No 213:

Question 21:

Draw a ray diagram to show how a convex lens can be used as a magnifying glass.

Answer:

If an object is placed between the focus and the optical centre of a convex lens, the image produced is virtual and erect. Thus, the lens works as a magnifying glass.

Page No 213:

Question 22:

Draw ray diagrams to show image formation by a convex lens in the following cases.
(a) the object is at 2F
(b) the object is between F and 2F
(c) the object is beyond 2F

Answer:

(a) The object is at 2F.

A real, inverted image is formed at 2F.

(b) The object is between F and 2 F


A real, inverted, magnified image is formed beyond 2F.

(c) The object is beyond 2F.

A real, inverted, diminished image is formed between F and 2F.



Page No 214:

Question 23:

A convex lens has a focal length of 10 cm. Column I gives you the distance of an object from the lens in different cases. Match the distance of the images given in column I with the cases in column I.

Distance of Object Distance of Image
a. 2 metres from lens i. 15 cm from lens on the same side
b. 30 cm from lens ii. between 10 and 20 cm from lens
c. 20 cm from lens iii. at about 10 cm from lens
d. 15 cm from lens iv. more than 20 cm from lens
e. 6 cm from lens v. at 20 cm from lens

Answer:

Distance of Object Distance of Image
a. 2 metres from the lens iii. About 10 cm from the lens
b. 30 cm from the lens ii. Between 10 cm and 20 cm from the lens
c. 20 cm from the lens v. 20 cm from the lens
d. 15 cm from the lens iv. More than 20 cm from the lens
e. 6 cm from the lens i. 15 cm from the lens on the same side

Page No 214:

Question 24:

Differentiate between:
(a) real and virtual images
(b) long sight and short sight
(c) microscope and telescope

Answer:

(a) Real and virtual images

            Real Image          Virtual Image
A real image can be captured on a screen. A virtual image cannot be captured on a screen.
A real image is always inverted. A virtual image is always erect.
Light rays meet to form a real image. Light rays do not meet to form a virtual image.

(b) Long sight and short sight
Long-Sightedness Short-Sightedness
This defect of vision is known as hypermetropia. This defect of vision is known as myopia.
In this defect, a person cannot see nearby objects clearly, though he/she can see distant objects clearly. In this defect, a person cannot see distant objects clearly, though he/she can see nearby objects clearly.
This defect is caused by the low converging power of the eye lens. It can also occur if the eyeball is too short. This defect is caused by the high converging power of the eye lens. It can also occur if the eyeball is too long.
In this defect, the image is focused behind the retina. In this defect, the image is focused in front of the retina.
This defect is corrected by using spectacles having convex lenses. This defect is corrected by using spectacles having concave lenses.

(c) Microscope and telescope
Microscope Telescope
Microscope is used to view things that are very near. Telescope is used to view things that are very far.
The focal point in case of a microscope is just a fraction of an inch off. The focal point in case of a telescope may be at a far-off place.
Microscope is used to look into small details like the structure of cells and unicellular organisms. Telescope is used to look large objects that are very far off (e.g., to look into space).

Page No 214:

Question 25:

Mention some uses of lenses in daily life.

Answer:

A lens is used in many optical instruments like a spectacle, magnifying glass, camera, film projector, microscope and telescope.

Page No 214:

Question 26:

Copy the table below into your notebook. Complete it by writing whether the image is real or virtual, magnified or diminished in each case.

Instrument Kind of Image Size of Image
a. camera    
b. slide projector    
c. eye    
d. magnifying glass    
e. microscope    
f. telescope    

Answer:

Instrument Kind of Image Size of Image
a. Camera  Real  Diminished
b. Slide projector  Real  Magnified
c. Eye  Real  Diminished
d. Magnifying glass  Virtual  Magnified
e. Microscope  Virtual  Magnified
f. Telescope  Real  Magnified

Page No 214:

Question 27:

Why is it important to take care of the eyes and go for a regular check-up?

Answer:

Early detection of the problem is necessary. It is important to take care of the eyes and go for regular check-ups to avoid or minimise vision problems.



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