Reflection of Light at Curved Surfaces Part-3

Lab Activity: 

Experiment to observe various types of images formed by a concave mirror and their characteristics

Aim: Observing the types of images and measuring the object distance and image distance from the mirror

Materials Required: A candle, paper, concave mirror (focal length should be known), V-Stand, tape or measuring scale.

Procedure:

• Place the concave mirror on V-stand, a candle and meter scale as shown in the below figure.
• Keep the candle at different distances from the mirror (say 10cm to 80cm) along the axis by moving the paper or screen.
• Find the position where you get the sharp image on the paper by taking care that flame should be above the axis of the concave mirror and paper should be below the axis.
• Note down your observation in the following table

Observation:

Observation No.	Distance of the candle from the mirror (Object Distance) u	Distance of the paper from the mirror (Image Distance) v	Bigger/Smaller Or Enlarged/Diminished than the object	Inverted or Erected

Result:

Images formed by a concave mirror for different positions of the object are classified as follows:

S.No.	Position of the object	Position of the image	Nature of the image
1	At Infinity	At F	Real, inverted and highly diminished
2	Between P and F	Behind the mirror	Virtual, erect and enlarged
3	At Focus F	At Infinity	Real, inverted and enlarged
4	Between C and F	Beyond C	Real, inverted and enlarged
5	At C	At C	Real, inverted and of the same size
6	Beyond C	Between F and C	Real, inverted and diminished

Questions related to the lab activity:

• Is it inverted or erect, enlarged or diminished?

Ans:    See column no.3 in the above tabular column to find the nature of the image for all types of images formed.

• What do you infer from the above table?

Ans:    From the above table, we can infer that images can be formed at positions different from the focus.

• Why only at point A?

Ans:    If we hold the screen at any points before or beyond point A, we see that the rays will meet the screen at different points. Therefore the images of the tip of the flame will be formed at different points due to the rays. If we draw more rays emanating from the same tip we will see that at point A. They do not meet at point B

• Where is the base of the candle expected to be in the image when the candle is placed on the axis of the mirror?

Ans:    The base of the candle is expected to be on the principal axis in the image when the object is placed on the axis of the mirror.

• During the experiment, did you get any positions where you could not get an image on the screen?

Ans:    Yes, we do not get an image on the screen when the object is placed at a distance less than the focal length of the mirror.

Construction of Images using Ray Diagrams:

In order to obtain the image of an object, we need to know at least two incident rays whose paths (after reflection from the mirror) are known. Any two of the following rays can be taken as the convenient incident rays for locating the image.

1. An incident ray passing through the centre of curvature (C):

Concave Mirror:

    An incident ray passing through the centre of curvature of a concave mirror reflects back along the same path, without any deviation.

Convex Mirror:

    An incident ray that appears to move towards the direction of the centre of curvature is reflected back in the same path without any deviation.

2. An incident ray parallel to principal axis:

Concave Mirror:

    An incident ray parallel to the principal axis passes through the Focus after reflection.

Convex Mirror:

    An incident ray passing parallels to the principal axis after reflection appears to come from the Focus of the mirror.

3. An incident ray passing through the Focus:

Concave Mirror:

    An incident ray passing through the Focus is reflected parallel to the principal axis.

Convex Mirror:

    An incident ray after reflection appears to come from the Focus of the mirror and is reflected parallel to the principal axis.

4. A ray of light incident at the pole of the mirror:

Concave Mirror:

    A ray of light incident at the pole of the mirror gets reflected along a path such that the angle of incidence is equal to the angle of reflection. i.e., <i = <r

Convex Mirror:

    The above is also true for convex mirror. i.e., <i = <r

Images formed by Concave Mirror:

Different positions of the object and images formed by a concave mirror.

1. Object between Pole (P) and Focus (F) - Image at beyond the mirror:

• Object position: The object is placed between P and F
• Image Position: The image is formed behind the mirror
• Nature and Size of the image: The image formed virtual, erect and enlarged.

2. Object at Focus (F) - Image at Infinity:

• Object Position: The object is placed at F
• Image Position: The image is formed at infinity
• Nature and Size of the image: The image formed real, inverted and highly enlarged.

3. Object between F and C - Image at beyond C:

• Object position: The object is placed between F and C
• Image Position: The image is formed beyond C
• Nature and Size of the image: The image formed real, inverted and enlarged.

4. Object is at C - Image is also at C:

• Object position: The object is placed at C
• Image Position: The image formed is also at C
• Nature and Size of the image: The formed is of the same size as that of the object, which is real and inverted.

5. Object beyond C - Image between F and C:

• Object position: The object is placed beyond C
• Image Position: The image is formed between F and C
• Nature and Size of the image: The image formed real, inverted and diminished.

Images formed by Convex Mirror:

Different positions of the object and images formed by a convex mirror.

1. Object at Infinity - Image at F:

• Image Position: Image is formed at Focus (F) behind the mirror
• Nature and size of the image: virtual, erect and highly diminished

2. Object between infinity and pole of the mirror - Image between P and F:

• Image Position: Image is formed between P and F behind the mirror
• Nature and size of the image: virtual, erect and diminished

Think and Discuss Question:

• Do you get an image when the object is placed at F? Draw the ray diagram. Do the experiment.

Ans:    Yes, we get an image when the object is placed at F.

Ray diagram for the object and image formed is drawn below:

Experiment: After performing the experiment, the results are as follows:

• Object at Focus (F) and image at Infinity.
• Nature and Size: real, inverted and enlarged.