 Ray optics, Mirror and Lense for NEET, JEE 2018

Ray optics

In This Topic we would study

• Concave, Convex, Plane Mirrors.
• Lenses.
• Image formation.
• Frequently asked question of previous exams.

Some basic terms

Incident ray- A light ray which strikes to surface.

Angle of incidence- The angle between light of incident and Norman.

Reflected ray- Reflected incident ray from surface is called reflected ray.

Angle of reflection- Angle between reflected ray and Norman.

Normal- A perpendicular hypothetical line from surface.

Point of incidence– A particular point of surface where incident ray, reflected ray and norman have connected. Some important formulas

1-  R = 2F      F = ½ R

2-  Mirror  formula-    1/v + 1/u  = 1/f

3- Magnification of mirror =  I/o =  -v/u

4-  Total no. of image between two mirror =  360/angle – 1

Types of Mirrors

1. Plane mirror

2. Spherical mirror

A-  Convex mirror

B-  Concave mirror

Important Terms

Pole – center point between reflector side of mirror.

Center of curvature-Mirror made by a sphere so the center of sphere is called Center of curvature.

Radius of curvature- Distance between pole and center of curvature.

Principle axis- a direct line connected to pole and center of curvature.

Image formation

Real Image – Front of mirror

Virtual Image – Back side of mirror

Image formation in concave mirror

Formation of image depend on object position with respect to mirror.

Here is 6 position.

Tricks

1- When object going near to mirror, Size of image will increase and going away from mirror.

2- In concave mirror image always real, when object position on focus point or before focus point.

1-when object on infinite

Note-when a ray passed from c it reflect back without change direction

• Image formed in focus point
• Real image
• Reverse
• Very small image 2- When object on between infinite and center of curvature.

• Image formed between focus and center of curvature
• Real
• Reverse smaller than object 3- When object on center of curvature.

• Image also formed in center of curvature
• Real
• Reverse and same size of object 4- When object on between centre of curvature and focus.

• Image formed away from center of curvature
• Real
• Bigger than object and reverse 5- When object on focus

• Reverse and very bigger than object
• Image formed on infinite
• Real 6- When object on between focus and pole.

• Image formed back side of mirror
• Virtual
• Bigger than object Image formation in convex mirror

Two condition only

1- When object on infinite- image formed on focus, virtual, upright, smaller than object.

2- Anywhere before mirror- back side of mirror, virtual, upright, smaller than object. Sign conversion for measurement of distance

• We using positive sign in direction of incident ray negative sign opposite of incident rays.
• Positive sign above principal axis and negative below p. axis.
• Focus distance of concave mirror is negative and positive of convex  mirror. Previous year questions

1- Mirror in front of driver seat would be

A-   concave

B-   convex

C-   plane

D-   non of these

2- Mirror used by doctor for testing of ear, nose, mouth would be

A –    convex

B –    concave

C –    plane

D –    all above

3- Mirror used for shaving would be

A –     Concave

B –     Convex

C –     Plane

D –    A and B

4- Mirror for own face looking is

A –  Concave

B –  Convex

C –  Plane

D –  Both A and C

5- Image formed by a plane mirror

A-  Real

B-  Virtual

C – Both

D- Non of above

6- If two mirror placed on Angle of 45 then total number of image would be

A- 9

B- 7

C- 8

D- infinite

Total no of image = 360/Angle  – 1

7- For looking of full image of any object, size of mirror would be

A-  Double of object

B-  Half of object

C-  Same as object

D- Triple of object

8- If two plane mirror place parallel to each other than the total no of image would be

A-  2

B-  4

C-  0

D-  infinite

9- Mirror using in lamp of street light is

A- Convex

B- Concave

C- Plane

D- Both A and B

Answers- 1.b, 2.b, 3.a, 4.c, 5.b,  6.b, 7.b, 8.d, 9.a

Lenses Tricks

The rules of image formation in lenses are same as mirror but rules would be interchanged as following –

Rules of Concave Mirror = Rules of Convex Lenses

Rules of Convex Mirror = Rules of Concave Lenses

For any questions, write in comment box.

Best of Luck

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