An object is kept at a distance of 18 cm, 20 cm, 22 cm and 30 cm, from a lens of power + 5D.
(a) The focal length of the given lens is:
(1) 0.2 cm
(2) 20 cm
(3) 5cm
(4) 0.5 cm
Answer
Focal length of a lens = 1/Power
= ⅕ = 0.2 m
Note: focal length obtained from the power formula is always in metres .
Thus, the focal length of given lens is 20cm .
So, the correct answer is (ii)
(b) In which case or cases would you get a magnified image?
(i) 18cm
(ii) 20cm
(iii) 22cm
(iv) All the above
Answer
Since
- Focal length is 20 cm
- And focal length is positive
- It is a convex lens
Now let us go through the options.
-
(i) 18cm -
Object between F and P: The image produced is
virtual, erect
and
magnified.
- (ii) 20cm - Object at F: The image produced at F is real, inverted and magnified.
- (iii) 22cm - Object between C and F: The image will be real, inverted and magnified.
Since magnified image is being formed at all object distances.
Thus, the correct answer is (iv) all the above
(c) Which of the magnified images can we get on a screen?
(i) At 18 cm
(ii) At 18 cm and 22 cm
(iii) At 20 cm and 22 cm
(iv) At 22 and 30 cm
Ans.
As explained in question (b), for convex lens
-
The
object at 18 cm
forms a
virtual image
. This
cannot
be captured on a screen.
-
The
object at 20 cm
forms an
image at infinity
. Since the rays never meet, the image
cannot
be caught on a screen.
- Only the object at 22 and 30 cm form real images with rays that converge to form an image. Hence, these images can be captured on a screen.
Thus the correct answer is (iv) at 22 cm and 30 cm
(d) The power of a lens of focal length 1 metre is called as:
(i) Refractive index
(ii) 1 dioptre
(iii) Speed of light
(iv) Wavelength
Answer
(e) Name the lens/mirror used in film projectors and telescopes:
(i) Convex lens
(ii) Concave lens
(iii) Convex mirror
(iv) Concave mirror
Ans. Let us go through our options:
- A Concave lens or a convex mirror produce virtual images. These cannot be used in film projectors as images cannot be captured on a screen.
- A convex lens and concave mirror produce real, magnified images (everywhere except between F and P ) that can be captured on screen.
