Refraction Througha Prism

Question 1

What Is a prism

Answer 1

A prism is an optical medium bound by 3 rectangular faces and 2 triangular faces parallel to each other

Question 2

With respect to a prism what are
Refracting faces
Refracting edge
Refracting angle or angle of the prism
Base of the prism 
Principle section of the prism

Answer 2

The two highly polished rectangular faces of the prism are the refracting faces.
The edge common to the two is the refracting edge.
The angle between the refracting faces is the angle of refraction or the angle of the prism.
The third ground face of the prism is the base.
Any section of the prism perpendicular to the refracting edge is a principle section of the prism.

Question 3

What is the importance of principal section

Answer 3

In all ray diagrams drawn to describe refraction through a prism it is the principle section that is drawn
The incident refracted and emergent ray are in the principal section

Question 4

What happens when
Monochromatic ray passes through a prism
A ray of white light passes through a prism

Answer 4

The ray is deviated through a certain angle toward s the base of the prism.

Different colours are deviated through different extent and the incident ray is dispersed into its constituent colours

Question 5

Is a ray always deviated toward the base when it emerges out of a prism?

Answer 5

A ray is deviated toward she base if the medium surrounding the prism is rarer than the medium of the prism
That is the refractive index of the prism is greater than the refracting index of the surrounding medium
Otherwise it deviates towards the vertex

Question 6

How is the path of the refracted ray in the prism when the prism is in the minimum deviation position?

Answer 6

It is parallel to the base if it is equilateral or isosceles .

Question 7

When does a ray of light incident normally on the face ifs prism just emerge from the opposite face?

Answer 7

When the refracting angle is equal to critical angle

A =C

Question 8

When does a ray of light incident at grazing incidence on one face of the prism just emerge from the opposite face

Answer 8

When refracting angle is equal to twice the critical angle

A = 2C

Question 9

What is the maximum permissible angle for a rayon light to be transmitted through a prism ?

Answer 9

2C

Twice the critical angle

Question 10

What is a total reflection prism?
Mention some uses
Mention some optical instruments where it is used.

Answer 10

When a prism is used to reflect light it is called a total reflection prism. It is a right angled isocelles triangle glass prism of n =1.5.
It can be used to turn the image by 90 180 or make an inverted image erect
It is used in optical instruments like binoculars periscopes reflecting telescopes and camera viewfinders.

Question 11

What are the advantages of total reflection prisms over mirrors

Answer 11

1. Even with mirrors with very good silvering maximum amount of light that can be reflected is 90 % but total reflection prisms reflect 100% of incident light.
2. In thick plane mirrors multiple reflection occurs resulting in multiple images.this problem doesn’t arise with TRP as reflection takes place only atone surface.
3. A mirror requires silvering which tarnishes with constant use whole reflection prism doesn’t.
4. In mirrors where silvering is non uniform images formed are of poor quality. Images formed by TRPs are always of good quality

Question 12

What is dispersion?

Answer 12

The phenomenon of splitting a rayon white / composite light into its constituent colours / wavelegths is known as dispersion.

Question 13

What is a spectrum?

Answer 13

The band of colours or the orderly arrangement of wavelengths into which a composite ray of light is dispersed

Question 14

What is a dispersive medium?

Answer 14

A medium in which different colours of light travel with different speed

Question 15

What is
Pure spectrum
Impure spectrum

Answer 15

A spectrum in which different colours are distinctly seen without any overlapping is called pure spectrum
A spectrum in which different colours cannot be seen distinctly due to overlapping is impure spectrum.

Question 16

Whips instrument is used to obtain a pure spectrum?

Answer 16

Spectrometer

Question 17

What conditions must be satisfied to get a pure spectrum?

Answer 17

The slit placed in front of the source must be as narrow as possible.
The beam incident on the prism must be a parallel beam .
The prism must be placed in the position of minimum deviation.
The different coloured rays in the emergent beam must be focused to different points on the screen.

Question 18

What is a thin prism ?

Answer 18

A prism with a small refracting angle that is less than 10° is a thin prism

Question 19

What is angular dispersion ?

Answer 19

The angular dispersion between two colours /wavelengths is the difference between their deviations.

Question 20

What is the mean deviation ?

Answer 20

Mean deviation of any two colours is the mean of the deviations of the two colours.

Question 21

What is dispersive power?

Answer 21

The dispersive power of a medium for a pair of colours / wavelengths is the ratio of angular dispersion between them to the mean deviation
Denoted by ω

Question 22

What are the factors on which dispersive power of the material of the prism depend on?

Answer 22

Only on the nature of the material

Question 23

State the conditions for dispersion without deviation.

Answer 23

Their refracting angles are on the inverse ratio of their refractive ties for the mean ray
Their refracting edges are pointing in opposite directions.