## Ray Optics and Optical Instruments  Important Questions for CBSE Class 12 Physics Reflection, Refraction and Dispersion of Light

1.Ray Optics or Geometrical Optics In this optics, the light is considered as a ray which travels in a straight line. It states that for each and every object, there is an image.

2.Reflection Reflection is the phenomenon of changing the path of light without any change in the medium.

3.Reflection of Light The returning back of light in the same medium from which it has come after striking a surface is called reflection of light.

4.Laws of Reflection

Two laws of reflection are given as below:

(i) The angle of incidence i is equal to the angle of reflection r.

(ii) The incident ray, reflected ray and normal to the reflecting surface at the point of incidence all lie in the same plane.

5.  Total number of images formed by two plane mirrors inclined at an angle 0 with each other is given by

6.Reflecting surface of a spherical mirror is a part of a hollow sphere. Spherical mirrors are of two types, (i) Concave spherical mirror (ii) Convex spherical mirror.

7.Sign Convention All measurements should be taken from pole of mirror. All measurements along the direction of incident ray will be positive and opposite to incident ray are negative. All the measurements for the distances above the principal axis are taken as positive and below the principal axis are taken as negative.

8.For a real object, u is negative whereas v is negative for real image and positive for virtual image.

9.Mirror Formula Mirror formula is a relation between focal length of the mirror and distances of objects and image from the mirror.

where, f = focal length, u = distance of the object from mirror, v = distance of the image from mirror.

10.Focal length of mirror (f) = Radius of curvature (R)/2

f=R/2

11.Linear Magnification The ratio of the size of the image formed by a spherical mirror I to the size of the object O is called the linear magnification produced by the spherical mirror.

where, I  = height of image and O = height of object

12.Magnification (m) It is negative corresponding to real image and positive for virtual image.

13.Refraction The phenomenon of changing in the path of light as it goes from one medium to another is called refraction.

14.Laws of Refraction

Two laws of refraction are given as below:

• The incident ray, refracted ray and the normal to the refracting surface at the point of incidence lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for the two given media. This constant is denoted by n and is called the relative refractive index.

n =sin i/sin r

where, n is refractive index of the second medium when first medium is air.

15.Refractive index of medium 2 w.r.t. medium 1 is denoted as 1n2 and defined as

17.Total Internal Reflection (TIR) When a ray of light travelling from denser medium to rarer medium is incident at the interface of two medium at an angle greater than the critical angle for the two media, the ray is totally reflected back to denser medium. This phenomenon is called Total Internal Reflection. It occurs only when angle of incidence in denser medium is greater (not equal) than critical angle, i.e. i> ic.

18.Principle of reversibility of light states that when final path of a ray of light after any number of reflections and refractions is reversed, the ray retraces its entire path.

22.Optical fibre, mirage, sparkling of diamond, totally reflecting prism, etc. work on the principle of total internal reflection.

24.Lens is a transparent medium bounded by two surfaces of which one or both surfaces are spherical.

(i) Convex or Converging Lens A lens which is thicker at the centre and thinner at its end is called convex lens.

Convex lenses are of three types which are given as below

(ii) Concave or Diverging Lens A lens which is thinner at the centre and thicker at its ends is called a concave lens.

Concave lenses are of three types which are given as below:

### Previous Years Examination Questions

1.A biconvex lens made of a transparent material of refractive index 1.25 is immersed in a water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason.  [All India 2014]

Ans. When a lens is placed in a liquid, where  refractive index is more than that of the material of lens, then the nature of the lens changes. So, when a biconvex lens of refractive index 1.25 is immersed in water (refractive index 1.33), i.e. in the liquid of higher refractive index, its nature will change. So, biconvex lens will act as biconcave lens or diverging lens.

2.A biconvex lens made of a transparent material of refractive index 1.5 is immersed in a water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason.  [Ail India 2014]

Ans. A biconvex lens acts as a converging lens in air  because the refractive index of air is less than that of the material of the lens. The refractive index of water is less than the refractive index of the material of the lens (1.5). So, its nature will not change, it behaves as a converging lens.

3.A convex lens is placed in contact with a plane mirror. A point object at a distance of 20 cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens? [All India 2014]

Ans. The adjacent figure shows a convex lens L in contact with a plane mirror P is the point object kept in the front of this combination at a distance of 20 cm from it.

Since, the image is coinciding with the object itself, the rays from the object after refraction from the lens fall normally on the mirror M and form an image coinciding with the object itself. So, the image is formed at the focus of the lens. So, focal length of the lens is 20 cm.

4.Write the relationship between angle of incidence i, angle of prism A and angle of minimum deviations from a triangular prism. [Delhi 2013]

Ans.The relation between the angle of incidence /’, angle of prism, A and the angle of minimum deviation, Am for a triangular prism is given by

5.How does focal length of a lens  change when red light incident on it is replaced by violet light? Give reason for your answer.                       [Foreign 2012]

Ans.The refractive index of the material of a lens increases with the decrease in wavelength of the incident light. So, focal length will decrease with decrease in wavelength according to the formula.

Thus, when we replace red light with violet light then due to increase in wavelength the focal length of the lens will decrease

6.Name the physical quantity which  remains same for microwaves of wavelength 1 mm and UV-radiation of 1600 A in vacuum.     [Delhi 2012]

Ans. Both microwave and UV-rays are a part of the electromagnetic spectrum. Thus, the physical quantity that remains for both types of radiation will be their speeds equal to c.  c=3×108m/s

7.Under what condition, does a  biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?   [Delhi 2012]

Ans. When refractive index of lens is equal to the  refractive index of liquid.

8.For the same value of angle of  incidence, the angles of refraction in three media A, B and C are 15 °, 25 ° and 35 ° respectively. In which medium, would the velocity of light be minimum? [All India 2012]

9.When monochromatic light travels from one medium to another, its wavelength changes but frequency remains the same. Explain. [Delhi 2011]

Ans.Because refractive index for a given pair of  media depends on the ratio of wavelengths and velocity of light in two medium and not on frequency

10.The refractive index of diamond is  much greater than that of glass. How does a diamond cutter make use of this fact?                 [HOTS; All India 2011C]

Ans. The refractive index of diamond is much higher than that of glass. Due to high refractive index, the critical angle for diamond air interface is low. The diamond is cut suitably so that the light entering the diamond from any face suffers multiple total internal reflections at the various surfaces.

11.If a ray of light propagates from a rarer to a denser medium, how does its frequency change? [All India 2011c]

Ans. Frequency remains unchanged when light  travels from one transparent medium to another transparent medium.

12.State the criteria for the phenomenon of total internal reflection of light to take place.  [Delhi 2011,2010, 2008C]

Ans. Following are the criteria for total internal reflection

(i) Light must pass from a denser to a rarer medium.

(ii) Angle of incidence must be greater than critical angle

13.A lens behaves as a converging lens in air and a diverging lens in water (g = 4/3). What will be the condition on the value of refractive index (g) of the material of the lens? [Delhi 2011c]

Ans. Refractive index of the material of lens is less than the refractive index of water.

14.A converging lens axially in contact  with a diverging lens; both the lenses being of equal focal lengths. What is the focal length of the combination?      [All India 2010]

15.A glass lens of refractive index 1.45 disappears when immersed in a liquid  What is the . value of refractive index of the liquid?  [Delhi 2010]

16.Calculate the speed of light in a medium whose critical angle is 30 °.  [Delhi 2010]

17.Why does the sky appear blue?  [Foreign 2010]

18.Under what condition does the formation of rainbow occur?  [All India 2010C]

Ans. Availability of rain drops causes refraction, dispersion and total internal reflection of sun light results in the form of rainbow and the back of the observer should be towards the sun.

19.Two thin lenses of power + 6 D and -2D are in contact. What is the focal length of the combination?  [All India 2010]

20.Two thin lenses of power + 4 D and -2D are in contact. What is the focal length of the combination?  [All India 2010]

21.Two thin lenses of power + 5D and -2.5D are in contact. What is the focal length of the combination?  [All India 2010]

22.Why are convex mirrors used as side view mirrors in cars? [Delhi 2009c]

Ans. Because convex mirror forms virtual, erect and smaller image of object irrespective of relative position of object from mirror and therefore, its field of view is very wide.

23.A converging lens of refractive index 1.5 is kept in a liquid medium having same refractive index. What would be the focal length of the lens in this medium? [hots; Delhi 2008]

24.How does the power of convex lens vary if the incident red light is replaced by violet light? [Delhi 2008]

25.Explain giving reason why the sun looks reddish at sunrise or sunset?  [Foreign 2008]