Light: Reflection and Refraction

Ans. Image is virtual, erect, laterally inverted and of same size as object.

Ans. Spherical mirror is a part of a sphere. If reflection takes place from inside, it is said to be concave mirror, and if the reflection takes place from outside surface it is a convex mirror.

Ans. a) The midpoint of mirror is known as pole.

b) The centre of curvature of a spherical mirror is the centre of that sphere of which mirror is a part.

c) The distance between pole and centre of curvature is called radius of curvature of the mirror.

d) The straight line joining the pole and centre of curvature is called principal axis.

e) The point on the principal axis through which parallel rays to the principal axis passes or appear to pass after reflection.

f) The diameter of the mirror or size of the mirror is called aperture.

g) The distance between focus and pole of a mirror is the focal length of the mirror.

Ans.

Advantage of Convex Mirror: It provide wider view of the rear traffic.

Disadvantage: It does not give the correct distance of the vehicle at the rear

Ans. If the focal length is measured in meter then the unit of power of a lens is diopter.

Ans. Convex lens are used in (i) spectacles, (ii) telescopes, (iii) microscopes, (iv) camera

Ans. Concave lenses are used in (i) shaving mirror, (ii) flash lights

Ans. A prism is an optical device with two triangular bases along with three rectangular lateral surfaces commonly inclined at an angle of 60 degrees.

Ans. The bouncing back of a ray of light in the same medium after striking on a surface of an object.

Ans. a) The angle between incident ray and normal at the point of incidence is called angle of incidence ∠i.

b) the angle between the reflected ray and normal at the point of incidence is called angle of reflection ∠r.

c) A plane containing, incident ray, normal, and reflected ray is known as plane of incidence.

Ans. Laws or reflection

a) The incident ray, the reflected ray and the normal at the point of incidence are always in the same plane.

b) Angle of incidence is equal to the angle of reflection.

Ans. The image of the laterally inverted word will be produced as an erect image and the person seeing it in rear view mirror as AMBULANCE.

Ans. The radius of curvature of a plane mirror is infinite.

Ans. The images formed by two parallel plane mirrors will be infinite.

Ans. Concave mirror.

Ans. Focal length f = R/2 = 20/2 = 10 cm

Ans. Speed of light in glass

V = c/n = (3 x 10⁸)/1.5 m/s = 2 x 10⁸ m/s

Ans. The meaning of refractive index of diamond is 2.42 is that speed of light in diamond is ½.42 times the speed of light in air/vacuum D,

i.e., speed of light in diamond

v = c/n = (3 x 10⁸)/2.42 = 1.24 x 10⁸ m/s

Ans. The image will be real, inverted and will be formed at centre of curvature of the lens.

Ans. a) If the image formed is erect, of same size and at equal distance behind the mirror as object and is in front of the mirror, then mirror is plane.

b) If the image is of larger size and erect and changes to inverted and smaller size by changing the position of mirror from the object it is concave mirror.

c) If the image is smaller and erect for all positions of mirror from the object, it is a convex mirror.

Ans. a) If a ray travels from one medium to another medium of same refractive indices then it will not bend and travels in a straight line.

b) Change in speed of light is the cause of refraction.

Ans. i) Face must be placed in between the pole and focus of the mirror so that enlarged, erect image of face can be formed.

ii) At focus, because rays coming from the focus after reflection will become parallel.

b) It is a combination of convex mirror (small head) and plane mirror (legs of normal size).

Ans. A convex lens of small focal length can be used as magnifying glass. When an object is placed in between pole and focus of the convex lens then enlarged, erected and same side of object image is formed.

Ans. Given, concave mirror of f = 15 cm

a) When object is placed at a distance of 10 cm from mirror (between P and f) image is virtual, enlarged and erected.

b) For object in between 2F and ∞ i.e., 40 cm image is inverted, diminished and real.

c) For object in between F and C (20 cm), image is inverted, enlarged and real beyond 2F (C).

d) For object at 2F (30 cm), the image is of same size.

Ans. (a) When a ray of light passes from one medium to another it bends from its path. The phenomenon of bending a ray of light when it passes from one medium to another is called refraction of light.

(b) In a lens if the image distance is negative then its meaning is that image formed is at the same side of object, virtual and erect. The is concave lens.

Ans. We require minimum two rays for locating the image formed by a concave mirror.

Ans. (a) 1. Concave mirror

2. Convex mirror

3 Concave parabolic

Ans. Power of a lens is the ability of the lens to converge or diverge a ray of light incident on it. It is the reciprocal of the focal length of the lens, ie., P = 1/f. The SI unit of power is D (dioptre) if f is measured in metre.

Ans. As n = c/v = sin i/sin r => v = c sin r/sin i

For same incident angle, v will be minimum for minimum value of angle r i.e., medium R.

Ans. Absolute refractive index of a given medium, w.r.t air/vacuum is known as absolute refractive index of the medium.

n = Speed of light in vacuum or air/Speed of light in medium

n = c/v as c > v

n > 1

= Velocity of light in B/Velocity of light in A = v_b/v_a = n_a/n_b

Higher the optical density, lesser is the velocity of light in the medium.

Ans. Given n = 4/3, c = 3 x 10⁸ m/s

v = ?

n = c/v => v = c/n

v = (3 x 10⁸ x 3)/4

= 2.25 x 10⁸ m/s

Ans. Radius of curvature of a spherical mirror is the radius of the sphere of which mirror is a part. It is the distance between pole and centre of curvature of a mirror. The radius of curvature is equal to the twice the focal length

R = 2f

Ans.

Ans. f = 1/P m

= 1/+1.5 m

= + 10/15 m

=+ 2/3 m

f = +66.7 cm

+f means the lens is convex (converging) lens.

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Ans. Midpoint of a lens through which a ray of light passes undeviated is known as optical centre.

A straight line joining two centre of curvatures of the lens is called principal axis of lens.

Ans. Image is formed at infinity, image will be real, inverted and enlarged.

Ans. Magnification m of a mirror gives the relative extent to which the size of image of an object is magnified w.r.t size of object. It is the ration of size of image (h₂) to size of an object (h₁)

m = Size of image /Size of object = h₂/h₁ = -v/u

Ans.

1/f = 1/v – 1/u

Ans.

Ans. It is a convex mirror. Ray diagram for any position of object.

Ans. Laws of refraction:

a) The incident ray, refracted ray and normal to the point of incidence all lie in the same plane.

b) The ratio of sin of incident angle to sin of angle of refraction for a given pair of medium is constant.

Sin i/Sin r = constant

Absolute refractive index of a medium is the ratio of speed of light in air or vacuum to speed of light in the medium.

Absolute refractive index = Speed of light in air/vacuum/Speed of light in medium

Ans. When a ray passes from optical rarer to denser medium, it bends towards the normal and vice versa.

∠i = angle of incidence

∠r = angle of refraction

∠e = angle of emergence

d = lateral displacement

Ans. a) For the same angle of incidence: The refractive ray in medium I deviates less than in medium II. Hence medium I is more denser than medium II

b) Refractive index of diamond = 2.42

Speed of light in air = 3 x 10⁸ m/s

Speed of light in diamond = ?

n = Speed of light in air/Speed of light in diamond = c/v

v = c/n = 3 x 10⁸/2.42

v = 1.24 x 10⁸ m/s

Ans. In observation (iii) of the table given u = 30 cm, v = 30 cm, which is only possible when u = v = 2f (at C)

2f = 30 or f = 15 cm

Ans. Power of a lens is the ability of a lens to bend a ray of light incident upon it. It is equal to the reciprocal of the focal length of the lens.

P = 1/f

If f is measured in m then unit of power is dioptre (D)

(b) v = +50 cm

m = 1

P = ?

Since, m = 1, m = v/u = 1

Or u = v = 50 cm

i.e., v and u both are at centre of curvatures C = 2f = 50 or f = 25 cm.

P = 1/f

P = +100/25 D

P = + 4D

Ans. a) When a ray of light falls normally on a glass slab then I = 0, and r = 0

b) vm = 1.5 x 10⁸ m/s

c = 3 x 10⁸ m/s

We know refractive index

n = c/vm

= 3 x 10⁸/1.5 x 10⁸

n = 2

Ans. a) More refractive index means more optical denser medium. Here refractive index of water is 1.33 and of alcohol is 1.36.

Hence alcohol is more optical denser than water.

b)

Ans. Given u = -12 cm

f = +15 cm

h₁ = 4.5 cm

v = ?, m = ?

Mirror formula

1/v + 1/u = 1/f

1/v = 1/f – 1/u = 1/5 – 1/-12 = 1/15 + 1/12

v = + 6.6 cm

m = -v/u = -6.6/-12 = 0.55

Size of image is smaller than size of object.

(a) Snell’s law = sin i/sin r = constant (Refractive index of the medium)

(b)

Ans. Given

v = -60 cm

u = -15 cm

a) Since the image is real the mirror is concave

b) m = -v/u = -(-60)/(-15) = -4

m = -4

c) Distance between object position and image position = 60 – 15 = 45 cm

d)

Ans. When an object is moved from infinity toward the pole of a concave mirror then its image moves from focus to infinity.

Ans. Snell’s laws of refraction

a) Incident ray, refracted ray and normal to the point of incidence lie in the same plane.

b) sin i/ sin 4 = ½ n

Ans.

(a) Distance between pole and centre of curvature is known as radius of curvature or the radius of the sphere of which mirror is a part is called radius of curvature.

(b) Focal length of a mirror is half of the radius of curvature

f = R/2

Ans.

Given object height h = + 5cm for a concave lens

u = -20 cm, f = -10 cm, v = ?

1/f = 1/v – 1/u

1/v = 1/f + 1/u

1/v = 1/-10 + 1/-20 = -3/20

V = -20/3 cm = -6.66 cm

Image formed is virtual, erect

M = h’/h = v/u

H’ = h x v/u = 5 x (-20/3 x 1/-20) cm

= 5 x 1/3 cm = 5/3 cm = 1.66 cm

Ans. Only in convex mirror, for all positions of the object placed in front of it is always virtual, erected and diminished. Hence this mirror is convex mirror.

Convex mirrors are used in automobiles as a rear view mirror because of wider field of view and formation of erect image.

Ans. a) Convex lens when object is in between F and C(2F)

b) Concave mirror when object is in between P and F its enlarged, erected and virtual image is formed.

Ans. Given f= +10 cm, u= ?

For virtual image

m = +2

As m = v/u or v/u = 2

v = 2u ———- (1)

1/v – 1/u = 1/f ———— (2)

Substituting (1) in (2)

1/2u – 1/u = 1/10

-1/2u = 1/10

u = -5 cm

For real image, f = 10 cm, m = -2

v/u = -2, v = -2u

1/v – 1/u = 1/f

1/-2u – 1/u = 1/10

-3/2u = 1/10

U = -15 cm

Ans. If the lower half of the lens is covered even then it will form a complete real, inverted image of same size at C₂(2F₂) with reduced intensity of image.

b) There will be no change in the nature and position of the object except in later case the image will be brighter.

Ans. Power of lens is the ability of a lens to converge or diverge light rays passing through it. It is the reciprocal of the focal length.

S.I unit: If focal length is measured in metre then unit of power of a lens is Dioptre (D)

P = 1/f (D)

Power of first lens: Focal length = +40 cm. Focal length is positive hence it is a convex lens.

P₁ = +100/f (cm) = +100/40 cm = +2.5 D

Power of second lens

Focal length = -20 cm

Its focal length is negative hence it is a concave lens

f₂ = -20/100 m = -1/5 m

P₂ = 1/f₂ = -5D

Ans. Laws of refraction:

a) The incident ray, refracted ray and normal to the point of incidence, all lie in the same plane.

b) The ratio of sine of incident angle and sine of angle of refraction for a given pair of medium is constant.

Sin i/sin r = constant

Absolute refractive index of a medium is the ratio of speed of light in air or vacuum and speed of light in the medium.

Absolute refractive index

= (speed of light in air/vacuum)/(speed of light in medium)