Plus Two Physics Chapter Wise Previous Questions Chapter 9 Ray Optics and Optical Instruments

Plus Two Physics Chapter Wise Previous Questions Chapter 9 Ray Optics and Optical Instruments is part of Kerala Plus Two Physics Chapter Wise Previous Questions and Answers Kerala. Here we have given Plus Two Physics Chapter Wise Questions and Answers Chapter 9 Ray Optics and Optical Instruments.

Kerala Plus Two Physics Chapter Wise Previous Questions Chapter 9 Ray Optics and Optical Instruments

Question 1
A ray of light falls on the side of a prism ABC whose refracting angle is A. The angles of incidence and refraction at the first face AB is i₁ and r₁. while at the second face AC is i₂ and r₂ (March – 2009)

a) Show that r₁ + r₂ = A
b) What do you understand by the critical angle of the prism.
c) Ifthe refractive index of the material ofthe below prism is 1.5, Find the critical angle ofthe prism.
d) Find the angle of incidence in orderthat the emergent ray may just graze the other side. (i₂ = 90°) take A = 60°
Some useful trigono metric values:
Sin 18°11 = 0.3121
Sin 41°49 = 2/3 = 0.6667
Sin 27°55 = 0.4682
Answer:
a) In quadrilateral AQNR
< A + < QNR = 180° …………………….. (1)
From ∆QNR

b) Critical angle of prism is the angle of incidence in the material of prism (denser medium) for which angle of refraction in the rarer medium is 90.

When angle of incidence becomes more than critical angle, total internal reflection takes place.

d) The emergent ray grazes the other side when i₂ = 90D hence r₁ + r₂ = 60, r, = 60 – C = 18°11

Question 2.
Combination of lenses are used in optical instruments to obtain the required power. (March – 2010)
a) What do you mean by power of a lens ? Express its unit.
b) Draw a neat schematic ray diagram forthe formation of image due to a point object placed in front of two thin convex lenses placed in contact.
c) Obtain an expression forthe effective focal length of the combination of two thin convex lenses in contact.
Answer:
a) The reciprocal of focal length in meter is called power.

c) Refer Section 9.5.4 Combination of thin lenses in contact

Question 3.
If a ray of light is incident on a medium of refractive index ‘μ₂’ from a medium of refractive index μ₁ (Say – 2010)
a) Under what conditions the ray of light gets total internally reflected for these media ?
b) Briefly explain how optical fibers transmit light signals.
Answer:
a) i) Light must travel from denserto rarer medium,
ii) Incident angle must be greaterthan critical angle.

Optical fibres consist of a number of long fibres made of glass or quartz (n = 1 .7). They are coated with a layer of a material of lower refractive index (1 .5). When light incident on the optical fibre at angle greater than the critical angle, it undergoes total in ternal reflection. Due to this total internal reflection, a ray of light can travel through a twisted path.

Question 4.
Refraction of light is the phenomenon of change in the path of light ray when it goes from one medium to another.
a) State the laws of refraction.
b) With the help of a ray diagram, derive lens makers formula for a convex lens of focal length ‘f’ as

‘μ’ is the refractive index of material of lens with respect to surrounding medium, R₁ and R₂ are the radii of curvatures of the surfaces of lens.

c) The radius of curvature of each face of a convex lens made of glass of refractive index 1.5 is 30cm. Calculate the focal length of the lens in air.
Answer:
a) First law
The incident ray, the refracted ray and the normal at the point of incidence are all in the same plane. Second law (Snell’s law)

The ratio of the sine of the angle of incidence to the sine of the angle of refraction ¡s a constant for a given pair of media and for the given colour of light used.

b) Consider a thin lens of refractive index n₂ formed by the spherical surfaces ABC and ADC. Let the lens is kept in a medium of refractive index n₁. Let an object ‘O’ is placed in the medium of refractive index n₁. Hence the incident ray OM is in the medium of refractive index n1 and the refracted ray MN is in the medium of refractive index n₂.

The spherical surface ABC (radius of curvature R₁) forms the image at l₁. Let ‘u’ be the object distance and ‘v₁’ be the image distance.

Then we can write,

This image I₁ will act as the virtual object for the surface ADC and forms the image at y.

Then we can write,

For convex lens,
f = +ve, R₁ = +ve, R₂ = -ve

Question 6.
A lens of a particular focal length is made from a given glass slab by adjusting radii of curvature. The formula used in this case is ens maker’s formula. (March – 2011)
a) Write lens maker’s formula.
b) Is ¡t possible for a given lens to act as a converging lens in one medium and a diverging lens in another medium? Why?
c) Compare the focal length of a given converging lens for blue light with that using red light. Are they equal or diferent? Why?
OR
A microscope is a device used to obtain magnified images of small objects.
a) Draw a labelled ray diagram of a compound microscope forming an image at the near point of the eye.
b) Obtain an expression for the magnification produced by a compound microscope.
c) If the objective lens is immersed in a transparent oil what will happen to the resolving power of the microscope? Expfain.
Answer:

b) Yes. Focal length depends on the refractive in dex of material of medium in which lens is placed.
e) Different. Refraction depends on the colour of light.
OR

The magnification produced by the compound microscope

Where m₀ & m_e are the magnifying power of objective lens and eyepiece lens

Where V₁ and u₀ are the distance of the image and object from the objective lens. Substituting (2) and (3) in (1), we get

for compound microscope, u₀ >> f₀ (because the object of is placed very close to the principal focus of the objective) and v₀ ≈ L, length of microscope (because the first image is formed very close to the eye piece.)

where L is the length of microscope, f₀ is the focal length of objective lens.

c) Resolving Power increases.

The above equation shows that, when refractive index (n) increases, the resloving power also in creases.

Question 7.
To derive the relevant formula for reflection by sphen cal mirrors and refraction by lenses we must adopt sign conventions for measuring distances. (Say – 2011)
a) State cartesian sign conventions used in ray optics.
b) Drawthe ray diagram forthe image formed bya concave lens.
c) Show that f = R/2 in the case of concave mirror, where f is the focal length and R is the radius of curvature.
d) Two lenses of powers +7D and -3D are combined. The focal length of the combination would be
a) -50 cm
b) +25cm
c) -25cm
d) +50 cm
OR
Raju passes a ray of yellow colour through an equilateral prism and the path of the ray is as shown in the following figure.

a) State the laws of refraction.
b) The refractive index of the material of the prism for yellow colour is 1.5. What would be the velocity of yellow colour through the prism? Velocity of light in vacuum is 3 x 10⁸ m/s.
c) Draw the path of the ray through the prism which suffers minimum deviation.
Answer:
a)

• According to this convention, all distances are measured from the pole of the mirror or the optical centre of the lens.
• The distances measured in the same direction as the incident light are taken as positive and those measured in the direction opposite to the direction of incident light are taken as negative.
• The heights measured upwards are taken as positive. The heights measured downwards are taken as negative.

Consider a ray AB parallel to principal axis incident on a concave mirror at point B and is reflected along BF. The line CB is normal to the mirror as shown in the figure.

Let θ be angle of incidence and reflection.


OR
a) 1) According to the first law of reflection, the angle of reflection equals the angle of incidence.
2) According to the second law of reflection, the incident ray, reflected ray and the normal to the point of incidence all lie in the same plane.

Question 8.
Sunlight gets scattered as it travels through earth’s atmosphere.
a) State Reyleigh’s Scattering law.
b) During sunset and sunrise the sun appears red. Explain. Why?
Answer:
a) The intensity of the scattered light from a molecule is inversely proportional to the 4m power of the wavelength.

b) At sunrise and sunset light has to travel a longer distance before reaching the earth. During this time, smaller wavelengths are scattered away. The remaining colours is red. Hence sky appears red in colour.

Question 9.
Using the data givetiHbelow, state as to which of the given lenses will you prefer to use as (March – 2012)
i) an eye-piece and
ii) an objective to construct an astronomical telescope? Give reason for your answer.

Answer:
i)  To increase magnifying power  of an astronomical telescope in normal adjustment, focul length (f_e) of eyepiece should be small (power of lens should be large). Hence lens L₄ should be used as eye-piece.

ii) When we use a lens of large focul length (f₀), we get a magnified image. Large aperture increase ‘resolving power and light gathering power of a tele scope. Hence we must use L₁ lens as objective lens of astronomical telescope.

Question 2.
A spherical surface of radius of curvature R, separates a rarer and a denser medium as shown in the figure.

a) Complete the path of the incident ray of light, showing the formation of a real image.
b) Derive the relation connecting object distance ‘u’, image distance ‘y’, radius of curvature R and the refractive indices n₁ and n₂ of the two media.
c) A thin convex lens of focal length 5 cm is used as a simple microscope by a person with normal near point (25 cm). What is the magnifying power of the microscope?
OR
In the figure PQ isa ray ncidenton a prism ABC.

Answer:
a) Complete the ray diagram showing the passage of light. Mark angle of incidence i, angle of emergence e, angle of deviation 3 and angle of retractions r₁ and r₂
b) Using the diagram obtain the relation δ = 1 + e – A
c) The critical angle for diamond is 30. What is its refractive index?
Answer:
b) Refer 9.5.1 Refraction at a spherical surface
b) Refer 9.6 Refraction through a prism

b) Refer 9.6 Refraction through a prism

Question 10.
The path of light rays through a convex lens when it is placed in two different media is shown in the figure. (Say – 2012)

a) What is the relation between the refractive indices μ, μ₁ and μ₂?
b) Find the focal length of the lens shown in the figure below. The radii of curvature of both surfaces are equal to R. (μ₁ < μ₂ < μ₃)
Answer:
a) for first Lens μ₁ = μ for second lens μ₁ > μ
∴ μ₁ = μ > μ₂
b) This lens can be devided into two half as shown in figure.

Question 11.
a) An equilateral glass prism is placed on a horizontal surface. A ray PQ is an incident on it. For minimum deviation: (March – 2013)

i) PQ Is horizontal
ii) QR is horizontal
iii) RS is horizontal
iv) None of these
b) A thick lens gives coloured images due to
c) In a compound micro scope the nature of the inte mediate image is
d) Based on refraction and total internal reflection explain the formation of rainbow.
a) QR is horizontal
b) Disperson
c) Real inverted and enlarged image
d) The rainbow is an example of the dispersion of sunlight by the water drops in the atmosphere.

The conditions for observing a rainbow are that the sun should be shining in one part of the sky while it is raining in the opposite part of the sky. There are two types rainbow (1) Primary rainbow (2) secondary rainbow.

Refer section 9.8.1. primary rainbow and secondary rainbow

Question 12.
When light rays entSrfrom one medium to another, refraction takes place. Consider refraction of light at a spherical surface separating two media of refractive indices ni and n2 (n₂ > n₁). (Say – 2013)
a) With the help of a ray diagram show the formation of image of a point object placed in the medium of refractive index n₁.
b) In the above case let the first medium be air and the second medium glass of refractive index 1,5R of spherical surface = 20cm. An object is placed in air 100cm from the glass surface. At what position the image is formed?
Answer:

Question 13.
a) Define power of a lens. What is its unit?
b) Consider two thin lenses of focal lengths f1 and f2 is contact. Obtain an expression for the effective focal length of this combination. What will be the power of this combination?
c) A convex lens of refractive index 1.5 has a focal length of 20cm in air. Calculate the change is its focal length when immersed in water of refractive index 4/3.
Answer:
a) Dioptre is the reciprocal of focul length enpressed in meteres. Unit of power is dioptre.
b) Obtain an expression forthe effective focal length of the combination of two thin convex lenses in contact.
or P = P₁ + P₂
where P is the power of the combination, P₁ and P₂ are the powers of the individual lenses.

Question 14.
Light has several properties like reflection, refraction etc. When light travels from an optically denser medium to a rarer medium? (March – 2014)
a) What happens to the light at the interface?
b) What are the technological applications of total internal reflection in nature? Briefly explain it.
c) What is the Brewster angle for air to glass transition? (Refractive index of glass is 1.5).
Answer:
a) The path of light deviate at interface ie, light undergoes refraction.
b) Brilliance of diamend is due to multiple total internal reflections. Total reflection prisms are right angled prisms based on total internal prisms. Optical fibres based on total internal reflection has many application in many fields like communication, medical diagnosis, optical diagnosis…. etc.
c) According to Brewster’s law

Question 15.
On hot summer days while moving in a bus the distant spot on highway appears to be wet even though it is not actually wet. (Say – 2014)
a) Which physical phenomenon is responsible for this effect.
b) What is this effect known as?
c) Draw the ray diagram showing this physical phenomenon.
d) How Snell’s law is modified in this situation?
Answer:
a) Total internal reflection
b) Mirage
c) On hot summer days the layer of air in contact with the sand becomes hot and rare. The upper layers are comparatively cooler and denser. When light raystravel from denser to rarer, they undergo total internal reflection. Thus image of the distant object is seen inverted. This phenomenon is known as mirage.
d)  where n is refractive index and C is the critical angle.

Question 16.
You are given two concave mirrors of different aperture sizes which one of the two you may select to get a sharper image.
a) i) One with a larger aperture size, ii) One with a smaller aperture size.
b) Why do you choose it?
Answer:
a) i) One with a larger aperture size
a) To collect more light

Question 17.
[The following is a choice question. Answer any one] (March – 2015)
A) A convex lens
a) is thicker at the edges than at the middle.
b) is thicker at the middle than at the edges.
c) diverges rays of light.
d) is of uniform thickness everywhere.

B) With the help of a ray diagram sketch the image formation of a convex lens when the object is between C and F.
OR
A) A ray of light travels from a denser to a rarer medium then, the ray
a) doesn’t bend at all.
b) bends towards the normal.
c) bends away from the normal.
d) goes along the normal.

B) Draw a diagram showing the path of a monochromatic light through a triangular prism.

Answer:

Question 18.
A) The following is a choice question: (March – 2016)
a) If the focal length of a double convex lens is 12 cm and radii of curvatures of faces are 10 cm and 15 cm respectively, what is the refractive index of the lens?
b) i) Draw the ray diagram showing the formation of image by a compound microscope.
OR
B) a) What is the structure of an optical fibre.
b) What is the principle used for transmitting audio and video signals using optical fibre? Explain the principle.
Answer:

OR
B) a) Each optic fibre consists of core and cladding. The refractive index of material of core is greater than that of cladding. The optical fibre is extremely thin and long strand made of fine quality glass or quartz.

b) Total internal reflection.
Refer 9.4 Total internal Reflection

Question 19.
The refraction of light through the atmosphere is responsible for many interesting phenomena. (Say – 2016)
a) How is the atmospheric refraction of sunlight affects the duration of a day?
b) A prism shown in the figure is designed to bend the rays by 180c.

Complete the ray diagram to show the image formation.
Answer:
a) Due to refraction, sun is visible before sunrise and after sunset. Hence duration of day increases.

Question 20.
The magnifying power of a telescope depends on the focal length of the objective and that of the eye piece.
a) Data of some lenses are given in the table.

Choose any two lenses which are to be preferred as objective and eye-piece to construct a telescope. Give reason for your selection.
b) A telesope has an objective of focal length 1 .44m and an eye-piece of focal length 006m. What is the seperation between the objective and the eye piece?
Answer:
a) L₂ Lens L₂ has large aperture. Hence it can collect large amount of light energy.
b) f₀ = 1.44m, fe = 0.06m.
The separation between objective and eye piece is f₀ + f_e.
f₀ + f_e = 1.44 + 0.06 = 1.50m

Question 21.
A concave lens always produces …………….. images (March – 2017)
i) real
ii) virtual
iii) magnified
iv) none of these
Answer:
ii) Virtual

Question 22.
a) Speed of light in glass is 2 x 10⁸ m/s. Refractive index of glass is ………………..
b) For an equilateral prism made of a material of refractive index , find the angle of minimum deviation for a ray of monochromatic light.
Answer:

Question 23.
A ray of light undergoes refraction when ¡t enters from air to glass. (Say – 2017)
a) Critical angle for glass – liquid boundary is sin’ (8/9). Determine the speed of light ¡ri liquid if speed of light in glasš is 2 x 10⁸m/s.
b) A picture is painted with blue, green and red colours. A glass slab is placed on this picture. Will the images of all these colours lie ¡n the same plane in the slab when viewed from the top of the glass slab? Why?
Answer:

b) Images of these colours will not be in same plane. Blue has more refractive index than red and green. Hence blue undergoes more apparent shift than red and green.

Question 24.
Derive the relation  for a glass prism.
OR
Derive lens makers formula for a convex lens producing a real image.
Answer:
Refer 9.6 Refraction through a prism
OR
Refer 9.5.2 Lens Maker’s Formula

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