RAY OPTICS

JEE MAIN CHAPTER OVERVIEW

Chapter Priority	3 (LOW)
Number of problems in PYQ	HIGH
Difficulty	MODERATE/HIGH

To solve JEE MAIN problems for Ray Optics, you need to remember all formulae given in these notes.

Note: If you don't remember the formulae for special cases given in these notes, problems in the exam will take a lot of time and will become calculation heavy.

KEY CONCEPTS FOR JEE MAIN

REAL & VIRTUAL OBJECTS and IMAGES

The following images depict the difference between real and virtual objects and images.

REAL OBJECT: When light rays actually diverge from a point before optical phenomenon, it is called a real object.

REAL IMAGE: When light rays actually converge at a point after optical phenomenon, it is called a real image.

VIRTUAL OBJECT: When light rays appear to come from a point, point before optical phenomenon, it is called a virtual object.

VIRTUAL IMAGE: When light rays appear to diverge from a point after optical phenomenon, it is called virtual image.

REFLECTION

Laws of reflection:

• Angle of Incidence = Angle of reflection

• Incident ray, Reflected ray and the Normal at the point of incidence all lie in the same plane

Often questions are asked on the laws of reflection as stated in the vector form.

We are asked to write the unit vectors in the directions of incident ray, reflected ray and normal and derive a relation between them. Given below is the result:

[The above result is very important and will help you save a lot of time in the exam. Whenever there are vectors given in a Ray optics question, this is the formula you need to use.]

Another common category of questions in JEE MAIN is when we either change the angle of incidence or the angle at which the mirror is kept while keeping the incident ray.

When we keep incident ray fixed but rotate the mirror, the reflected ray rotates in the same direction as the mirror by twice the angle rotated by the mirror.

When we keep the mirror fixed but rotate the incident ray, the reflected ray rotates by the same angle as the incident ray but in opposite direction to the incident ray.

ANGLE OF DEVIATION AFTER REFLECTION

A common question seen in JEE MAIN is to calculate the deviation after reflection from one or two surfaces.

When reflection is from a single mirror, the deflection is 180 - 2i (where i is angle of incidence).

When reflection is from two mirrors, the deflection is 360 - 2x (where x is angle between two mirrors).

Questions on Number of Images formed

Many times, questions are asked on how many images of an object will be formed after reflection from two surfaces. In this case follow the algorithm given above.

Note: All images formed will be on the dashed circle drawn in the image with center as the intersection of the mirrors as shown.

SPHERICAL MIRRORS

They are cut out from a hollow transparent sphere of radius R with one side silvered.

Note: We only analyse mirrors with small apertures in RAY OPTICS for JEE MAIN.

For Convex mirror, parallel rays coming from infinity after reflection appear to be diverging from a common point called Focus. For Concave mirror, parallel rays coming from infinity after reflection actually converge to a common point called Focus.

Focal length in both cases = Half of Radius.

SIGN CONVENTION

This is an extremely important topic for JEE MAIN and ADVANCED.

1. All distances are measured from the pole.

2. Distances measured in the direction of the incident ray are taken as positive.

3. Distances measured in the direction opposite to the incident ray are taken as negative.

4. Distances measured above the principal axis are positive.

5. Distances measured below the principal axis are negative.

MIRROR FORMULA

Short note on Magnification:

1. If m>0, then both image and object are on the same side of the principal axis

2. If m<0, then image and object are on opposite sides of principal axis

3. If |m| > 1, then image is larger than object

4. If |m| < 1, then image is smaller than object

5. If |m| = 1, then image is the same size as the object

VELOCITY OF IMAGE

WHEN OBJECT IS MOVING HORIZONTALLY

When velocity of object is positive, velocity of image is negative and vice versa.

WHEN OBJECT IS MOVING VERTICALLY

TRY A JEE MAIN PYQ

Q. The incident ray, reflected ray and the outward drawn normal are denoted by the unit vectors a, b and c respectively. What is the correct relation between the vectors?

[JEE MAIN 2021]

[Hint: See formula above]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

Q. A light ray is incident, at an incident angle 𝜃1 , on the system of two plane mirrors M1 and M2 having an inclination angle 75 degrees between them. After reflecting from mirror M1 it gets reflected back by the mirror M2 with an angle of reflection 30 degrees. The total deviation of the ray will be?

[JEE MAIN 2022]

[Hint: See formula above]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

Q. A point source of light S, placed at a distance 60 cm in front of the centre of plane mirror of width 50 cm, hangs vertically on a wall. A man walks in front of the mirror along a line parallel to the mirror at a distance 1.2 m from it (see in figure). The distance between the extreme points where he can see the image of the light source in the mirror is____cm

[JEE MAIN 2021]

[Hint: Draw ray diagrams connceting extreme points of the mirror.]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

Q. Car B overtakes another car A at a relative speed of 40 m/s . How fast will the image of car B appear to move in the mirror of focal length 10 cm fitted in car A, when the car B is 1.9 m away from the car A?

[JEE MAIN 2021]

[Hint: See formula above.]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

REFRACTION Laws of refraction:

1. The incident ray, the refracted ray and the normal to the interface at the point of incidence, all lie in the same plane.

2. The ratio of the sine of the angle of incidence to the sine of angle of refraction is constant. Remember that the angles of incidence (i ) and refraction (r ) are the angles that the incident and its refracted ray make with the normal, respectively.

	DENSER MEDIUM	RARER MEDIUM
Speed of light	Slower	Faster
Refractive Index	Higher	Lower
When light enters	Bends towards normal	Bends away from normal

Some important formulae which are directly asked in JEE MAIN

NOTE: It is RELATIVE permeability and permittivity in point 2 above, NOT permeability or permittivity of medium.

TOTAL INTERNAL REFLECTION

When light goes from denser to rarer medium, the angle of refraction is greater than angle of incidence. For a particular critical angle of incidence, the angle of refraction is 90 degrees. Beyond this angle of incidence, there is total internal reflection of light. Light ray gets reflected within the denser medium and does not enter the rarer medium.

REAL & APPARENT DEPTH

To an observer in a different medium, the apparent depth of an object is different from the actual depth of the object.

REFRACTION THROUGH A TRANSPARENT SLAB

When refraction takes place through a parallel glass slab of thickness 't', we can define two kinds of shift.

Apparent Shift = distance between the real position of the object and the image of the object formed after refraction

Lateral Shift = Parallel shift in the incident ray after refraction

REFRACTION THROUGH SPHERICAL SURFACES

LENSES

LENS MAKER'S FORMULA

Common Misconception:

1. Focal length of concave mirror is always negative

2. Focal length of convex mirror is always positive

Correct version:

1. Focal length of concave mirror is negative when surrounding medium is rarer than material of lens

2. Focal length of concave mirror is positive when surrounding medium is denser than material of lens

3. Focal length of convex mirror is positive when surrounding medium is rarer than material of lens

4. Focal length of convex mirror is negative when surrounding medium is denser than material of lens

POWER OF A LENS

1. Power of a lens is the reciprocal of its focal length.

2. The power of the lens is calculated in Diopters (D) if the focal length is given in meters.

3. A diverging lens has a negative optical power, while a converging lens has positive optical power.

Power of a combination of lenses

For thin lenses in contact, power of the combination of lenses is the sum of the power of all lenses (with appropriate signs).

SILVERING OF LENSES

When one side of a lens is silvered, it behaves like a spherical mirror.

Power of the Lens + Silvered Surface is given by: PCOMBINATION = 2PLENS + PMIRROR

LENS FORMULA

MAGNIFICATION (LENSES)

VELOCITY OF IMAGE IN LENSES

Vox = Velocity of object in x-dir

VIx = Velocity of image in x-dir

Voy = Velocity of object in y-dir

VIy = Velocity of image in y-dir

TRY A JEE MAIN PYQ

Q. The difference of speed of light in the two media A and B (VA - VB) is 2.6 × (10)^7 m/s. If the refractive index of medium B is 1.47, then the ratio of refractive index of medium B to medium A is : (Given : speed of light in vacuum c = 3 × 10^8 m/s)

[JEE MAIN 2022]

[Hint: See formula above]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

Q. Time taken by light to travel in two different materials A and B of refractive indices μA and μB of same thickness is t1 and t2 respectively. If t2 - t1 = 5 × (10)^(-10) s and the ratio of μA to μB is 1 : 2. Then the thickness of material, in meter is: (Given vA and vB are velocities of light in A and B materials respectively).

[JEE MAIN 2022]

[Hint: See formula above - How is refractive index related to velocity?]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

Q. A ray of light is incident at an angle of incidence 60 degrees on the glass slab of refractive index √3. After refraction, the light ray emerges out from other parallel faces and lateral shift between incident ray and emergent ray is 4√3 cm. The thickness of the glass slab is _____ cm.

[JEE MAIN 2022]

[Hint: See lateral shift formula above]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

PRISM

Minimum Deviation and Prism Formula

THIN PRISM

DISPERSION

When white light is passed through a glass prism it splits into its spectrum of colours (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colours is termed as dispersion.

This happens because different colours have different wavelength and refractive index is different for light of different wavelength.

Angular Dispersion, Mean Deviation & Dispersive Power

TRY A JEE MAIN PYQ

Q. The angle of deviation through a prism is minimum when: (A) Incident ray and emergent ray are symmetric to the prism (B) The refracted ray inside the prism becomes parallel to its base (C) Angle of incidence is equal to that of the angle of emergence (D) When angle of emergence is double the angle of incidence Choose the correct answer from the options given below:

[JEE MAIN 2021]

[If you cannot solve any doubts related to any chapter of physics, upload a photo of your doubt at www.savai.co.in or WhatsApp me @ 7982286138, we will send solution.]

Hope you can REVISE RAY OPTICS FOR JEE MAIN PHYSICS from this guide to answer all varieties of JEE MAIN PYQs.

For optical instruments, read this BLOG POST

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