Exams Prep Master
A ray or light ray is a straight or curved line that is perpendicular to the wavefront of light. There are many rays that are used to analyze an optical system. They are incident ray, reflected ray, and normal. An incident ray is a ray of light that strikes a surface, an angle formed between the incident ray and normal to the surface is the angle of incidence. A reflected ray is a ray of light that gets reflected back from the surface into the surroundings, an angle formed between the reflected ray and normal to the surface is the angle of reflection.
Very Short Answer Questions [1 Mark Questions]
Ques. What is the size of the image if the object is placed at infinity and position of the image is at the focus in concave lens ?
Ans: The size of the image will be highly diminished when the object is placed at infinity and position of the image is at the focus.
Ques. Where will be the position of image in convex lens when the object is placed at infinity and size of the image is highly diminished ?
Ans: Image will be formed at the focus (F), behind the mirror, virtual an erect image will be formed.
Ques. What is the position of object when image is placed between F and C and size of the image is diminished in concave lens ?
Ans: Object will be placed beyond the center of curvature (C) also real and inverted image will be formed.
Ques. Where will be the position of image when the object is placed between infinity and the Pole in convex lens ?
Ans: The imge will be formed between P and F behind the mirror also the size of the image will be diminished.
Ques. What is the size and nature of the image when the image is placed beyond C and position of the object is between C and F in concave lens ?
Ans: The size of the image will be Enlarged and nature of the image will be real and inverted.
Short Answer Questions [2 marks Questions]
Ques. What will happen to the image formed by a mirror if half of it is covered with black paper?
Ans: An object is a source of incident rays whereas reflected rays form an image. A real full image will be formed. A real image is formed when the light rays come from a source and form an image. If half of the mirror is covered with black paper then there will be no change in the formation of an image. But there will be a slight change in the intensity of the image, it reduces due to the covered paper.
Ques. When we read any book because of the light it reflects. Why do we not see a faint image of ourselves in the book?
Ans: As we know the fact that the image is formed because of the regular reflection of light. Whereas, when we read any books, magazines, or newspapers there is an irregular reflection of light or diffused reflection. This is the reason that we are not able to see any images of ourselves in the book.
Ques. In a telescope the focal length of the objective and the eyepieces are 70 cm and 7 cm respectively. What is magnification power and the tube length?
Ans: According to the question, the formula for the magnification power is
M = - fo/fe
Fo = 70 cm
Fe = 7 cm
M= -70/7 = -10 (power of magnification)
Tube length L = fo + fe = 70 +7 = 77 cm
Hence, the magnification power is -10 and the length of the tube is 77 cm.
Ques. In a camera, the focal length of the objective and the eye piece are 54 cm and 6 cm respectively. What is the magnification power and the tube length?
Ans: According to the question, the formula for the magnification power is
M = -fo/fe
Fo = 54 cm
Fe = 6 cm
M = -54/6 = -9
For Tube length: L = fo + fe = 54 +6 =60 cm
Hence, the length of the tube is 60 cm and the power of magnification is -9.
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Long Answer Questions [3 Marks Questions]
Ques. Thin prism of angle 90 degrees gives a deviation of 30 degrees. What is the refractive index of the material of the prism?
Ans: n= sin (A + dm/2)/ sin (A/2)
Putting the values as per given in the question:
A = 90, dm= 30
Now, N = sin (90 + 30/2)/ sin (90/2)
N = sin 60/ sin 45 = 0.667
Hence, the value of the refractive index of the material of the prism is 0.667
Ques. The prism of angle 120 degrees gives a deviation of 60 degrees. What is the refractive index of the material of the prism?
Ans: N = sin (A + dm/2)/ sin A/2
According to the question:
A= 120 degree
dm = 60
Putting these values in the formula:
N = sin (120 + 60/2)/ sin 120/2
Sin 90/ sin 60 = 1/ 0.86 = 1.16
Ques. What is the focal length of a convex lens of focal length 40 cm in contact with a concave length 30 cm.
Ans: According to the question,
Focal length of the convex lens (f1) = 40 cm
Focal length of the concave lens (f2) = -30 cm
Focal length of the lenses = f
The required formula of the focal length is: 1/f = 1/f1 + 1/f2
1/f = 1/40 – 1/30 = 3-4/120 = -1/120
Hence, the focal length of the lenses are 120 cm.
Ques. What is the focal length of a convex lens of focal length 50 cm in contact with a concave lens of focal length 25 cm.
Ans: According to the question,
Focal length of the convex lens (f1) = 50 cm
Focal length of the concave lens (f2) = 25 cm
Focal length of the lenses = f
The required formula of the focal lenses: 1/f = 1/f1 + 1/f2
1/f = 1/50 – 1/25 = 1-2/50 = -1/50
Hence, the focal length of the lenses are 50 cm.
Very Long Answer Questions [5 marks Questions]
Ques. The image of a small bulb fixed on the wall of a dining room is to be obtained on the opposite wall 2 m away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose?
Ans: As per the given data in the question:
Distance between the object and the image (d) = 2 m
A convex lens has a maximum focal length of ‘f max’
The maximum focal length will be f max = d/4
=2/4 = 0.5m
Hence, the required focal length of the convex lens is 0.75 m.
Ques. A van side mirror is placed 80 cm from an object. The image of the object on the van side mirror is formed by a convex lens at two different locations separated by 30 cm. Determine the focal length of the lens?
Ans: According to the question,
Distance between the van and the object: D= 80 cm
Distance between two locations: d = 30 cm
‘f’ is the focal length of the lens
f = (D*D – d*d)/ 4D
f = 6400 – 900/ 320 = 17.18 cm
Hence, the focal length of the convex lens is 17.18 cm
Ques. A policeman is standing 100 cm from an object with a mirror. The image of the object on the mirror is formed by a convex lens at two different locations separated by 40 cm. Determine the focal length of the lens.
Ans: The distance between the mirror and the object D is 100 cm
The distance between two locations of the convex lens d is 40 cm
‘f’ is the focal length of the lens
f = (D*D – d*d)/4D
f = 10000 – 1600/ 400 = 21 cm
Hence, the focal length of the convex length is 21 cm.
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