To Study the Image Formation due to Concave Mirror at Different Positions
Hello students we are familiar with the concepts of light when falls on concave mirror produces convergence and produces divergence when falls on convex mirror. Also we are familiar with the paths that are followed by reflected rays of light when incident from different directions. In this experiment we are going to discuss the activity/experiment to find the focal length of given mirror.
Objective of the experiment….
To study the image formation due to concave mirror at different positions
Instruments required for the experiment…..
Concave mirror, screen (we can a card board paper paste on wooden plate as a screen) , mirror stand, meter scale, narrow slit of light, and plane table.
Assumptions used for the experiment…..
When ray of light is made incident on the concave/convex mirror it follows specific paths,
- When ray of light is parallel to principal axis, it gets reflected back through focus.
- When a ray of light passes through the focus, it gets reflected parallel to principal axis.
If the rays of light pass through centre of curvature of mirror, it gets reflected back and traces the same path.
Formula used…..
According to sign conventions we need to assume the distance as follows,
- Object distance, u= -ve,
- Image distance, v = -ve/+ve (depends upon the position of image) and
- Focal length, f= -ve for concave mirror/+ve for convex mirror.
Diagram showing the experimental condition….
Experimental setup is shown below,
Fig.1) Experimental setup
Ray diagram showing the experimental conditions are as follows,
Case 1) Object kept at infinity
Case 2) Object kept between focus and centre of curvature
Case 3) Object kept beyond centre of curvature
Procedure to find the focal length….
- Make a hollow box using card board paper and make as small slit on it. Place small bulb with connecting wires and switch in it. This will be our source of light with slit.
- Place concave mirror on mirror stand, mark point P (pole) on the table. Now don’t move the mirror throughout the experiment.
- Place the screen on the table at certain distance from the mirror.
- Find the approximate focal length of mirror by focusing any object in the window on mirror and finding its image on the screen. Mark the position of screen on table as F, measure the distance between P and F record as ‘f’. This will be our approximate focal length.
- Now keep the slit of light at more than the double of focal length, i.e. beyond the centre of curvature. Measure this distance and record as ‘u1’
- Standing behind the mirror stand and looking in to screen, adjust the position of screen such that the image of slit of light can be observed clearly on the screen. Mark the position of screen on table as Q. Measure the distance of point Q from P and record it as ‘v1’
- Note down the nature of image i.e. inverted /erected, real /virtual, diminished /magnified.
- Change the position of slit from the mirror and repeat the same procedure and complete the observation table given below.
Observation:
Focal length of mirror (approximate value) =…….cm
Observation table…
Sr No. | Position of object from the mirror
And object distance from mirror (u) cm |
Position of image from the mirror (v) cm
And image distance from mirror (u) cm |
Nature of image
(Real/Virtual) |
Size of image
(magnified/ diminished) |
1 | Beyond centre of curvature
u1=……. |
v1=……. |
||
2 | Centre of curvature
u2= |
|||
3 | Between centre of curvature and focus
u3= |
v3= |
Results from the experiment…
- Position and nature of image when object beyond centre of curvature=………..
- Position and nature of image when object at centre of curvature=………..
- Position and nature of image when object between centre of curvature and Focus =………..
Precautions that should be followed while performing experiment….
- Position of our neck/eyes, mirror and screen should be collinear.
- Mark the positions accurately to measure the distances.
- Avoid the parallax error while observing the image of object/slit in the screen.
Some important questions that needs to revised for the experiment…
Q.1) What do you mean by convergence of rays
Answer: When a parallel beam of rays falls on the concave mirror, after reflection all the reflected rays pass through the same point, so it appears like all the rays meet at the same point on principal axis. This is known as convergence of rays.
Q.2) Define: 1) principal focus 2)radius of curvature 3)pole of spherical mirror.
Answer:
Principal axis: When beam of incident ray is parallel to principal axis, after reflection all the reflected rays’ gets passes through same point which lies on principal axis called as Principal focus.
Radius of curvature: The radius of the imaginary sphere of which the mirror is part is called as radius of curvature.
Pole: The central point lies on the mirror surface is known as pole of mirror.
Q.3) Define magnification due to mirror, write its formula.
Answer: The ratio of height of image to the height of object is known as magnification of mirror. It is give as,
Q.4) What are the rules of reflection for the concave mirror
Answer: The rules of reflection are
- If incident ray is parallel to principal axis, reflected ray passes through focus
- If incident ray passes through the focus, reflected ray becomes parallel to principal axis.
- If the incident ray passes through centre of curvature, traces same path after reflection.