CLASS 10 CBSE PHYSICS PRACTICAL

EXPERIMENT: Determination of the Focal Length of Concave Mirror and Convex Lens by Obtaining the Image of a Distant Object

AIM:

To determine the focal length of:

  1. Concave mirror
  2. Convex lens
    by obtaining the image of a distant object.

MATERIALS REQUIRED:

S. No.ApparatusSpecificationQuantity
1Concave mirrorFocal length ~15-20 cm1
2Convex lensFocal length ~15-20 cm1
3Mirror/Lens holder standWooden or metal stand2
4Screen (white paper/cardboard)Size: 15 cm × 15 cm1
5Screen holder/standAdjustable1
6Metre scale100 cm, least count 1 mm1

THEORY:

A. CONCAVE MIRROR:

Definition:
A concave mirror is a spherical mirror whose reflecting surface is curved inward (towards the center of curvature).

Important Terms:

  • Pole (P): The center point of the reflecting surface
  • Principal Axis: The line passing through the pole and center of curvature
  • Principal Focus (F): The point on the principal axis where parallel rays converge after reflection
  • Focal Length (f): The distance between the pole and the principal focus
  • Center of Curvature (C): The center of the sphere of which the mirror is a part
  • Radius of Curvature (R): Distance PC

Relationship:

R = 2f
(Radius of curvature = 2 × Focal length)

Principle for Distant Object:
When parallel rays from a distant object (object at infinity) fall on a concave mirror, they converge at the principal focus (F) after reflection.

Characteristics of Image:

  • Nature: Real and inverted
  • Size: Highly diminished (point-sized)
  • Position: At the focus (F)

Ray Diagram:



Distance P to Screen = f (Focal length)

B. CONVEX LENS:

Definition:
A convex lens is a transparent optical device that is thicker at the center and thinner at the edges, which converges light rays.

Important Terms:

  • Optical Center (O): The geometric center of the lens
  • Principal Axis: The line passing through the optical center perpendicular to the lens surface
  • Principal Focus (F): The point where parallel rays converge after refraction
  • Focal Length (f): The distance between the optical center and the principal focus

Principle for Distant Object:
When parallel rays from a distant object (object at infinity) pass through a convex lens, they converge at the principal focus (F) on the opposite side.

Characteristics of Image:

  • Nature: Real and inverted
  • Size: Highly diminished (point-sized)
  • Position: At the focus (F)

Ray Diagram:



Distance O to Screen = f (Focal length)

PROCEDURE:

PART A: FOCAL LENGTH OF CONCAVE MIRROR

Step-by-Step Method:

  1. Selecting a Distant Object:
  • Choose a well-illuminated distant object such as:
    • A distant tree
    • A distant building
    • A window grill
    • Sun (use with caution, never look directly)
  • The object should be at least 50 times the focal length away (preferably more than 10 meters).
  1. Setting up the Mirror:
  • Mount the concave mirror vertically on the mirror holder/stand.
  • Ensure the reflecting surface faces the distant object.
  • Place it near a wall or on a stable table.
  • The principal axis should be horizontal and directed towards the distant object.
  1. Positioning the Screen:
  • Take a white screen (paper mounted on cardboard).
  • Hold it between the mirror and the distant object.
  • The screen should be perpendicular to the principal axis.
  1. Obtaining Sharp Image:
  • Move the screen slowly back and forth along the principal axis.
  • Observe the image of the distant object on the screen.
  • Continue adjusting until you get the sharpest, brightest, smallest, and clearest inverted image.
  • The image should be well-defined with clear details.
  1. Measuring Focal Length:
  • Once the sharp image is obtained, mark or fix the screen position.
  • Measure the distance between the pole of the mirror (P) and the screen using a metre scale.
  • Place the metre scale along the principal axis.
  • Note the reading carefully to the nearest millimeter.
  • This distance = Focal length (f) of the concave mirror
  1. Repetition:
  • Repeat the experiment at least 3 times using different distant objects.
  • Record each measurement.
  • Calculate the mean focal length.

PART B: FOCAL LENGTH OF CONVEX LENS

Step-by-Step Method:

  1. Selecting a Distant Object:
  • Choose a well-illuminated distant object such as:
    • A distant tree
    • A distant building
    • Sun (use with caution)
  • The object should be far away (more than 10 meters).
  1. Setting up the Lens:
  • Mount the convex lens vertically on the lens holder/stand.
  • Ensure the lens faces the distant object.
  • The principal axis should be horizontal and directed towards the distant object.
  1. Positioning the Screen:
  • Take a white screen (paper mounted on cardboard).
  • Hold it on the opposite side of the lens from the distant object.
  • The screen should be perpendicular to the principal axis.
  1. Obtaining Sharp Image:
  • Move the screen slowly back and forth along the principal axis.
  • Observe the image of the distant object on the screen.
  • Continue adjusting until you get the sharpest, brightest, smallest, and clearest inverted image.
  • The image should be well-defined.
  1. Measuring Focal Length:
  • Once the sharp image is obtained, mark or fix the screen position.
  • Measure the distance between the optical center of the lens (O) and the screen using a metre scale.
  • Place the metre scale along the principal axis.
  • Note the reading carefully to the nearest millimeter.
  • This distance = Focal length (f) of the convex lens
  1. Repetition:
  • Repeat the experiment at least 3 times using different distant objects.
  • Record each measurement.
  • Calculate the mean focal length.

OBSERVATIONS:

Least Count of Metre Scale: 1 mm = 0.1 cm

A. FOR CONCAVE MIRROR:

Observation Table:

S. No.Distant Object UsedDistance between Pole (P) and Screen (cm)Focal Length f (cm)
1Distant tree18.218.2
2Distant building18.418.4
3Window frame18.318.3

Calculation:

Mean Focal Length of Concave Mirror:

f = (f₁ + f₂ + f₃) / 3

f = (18.2 + 18.4 + 18.3) / 3

f = 54.9 / 3

f = 18.3 cm

Mean Focal Length of Concave Mirror (f) = 18.3 cm

B. FOR CONVEX LENS:

Observation Table:

S. No.Distant Object UsedDistance between Optical Center (O) and Screen (cm)Focal Length f (cm)
1Distant tree15.815.8
2Distant building15.615.6
3Sun rays15.715.7

Calculation:

Mean Focal Length of Convex Lens:

f = (f₁ + f₂ + f₃) / 3

f = (15.8 + 15.6 + 15.7) / 3

f = 47.1 / 3

f = 15.7 cm

Mean Focal Length of Convex Lens (f) = 15.7 cm

RESULT:

  1. Focal length of the given concave mirror (f) = 18.3 cm
  2. Focal length of the given convex lens (f) = 15.7 cm

(Note: The above values are sample readings. Actual values will depend on the mirror and lens used.)

PRECAUTIONS:

General Precautions:

  1. Apparatus Setup:
  • Ensure the mirror/lens is mounted vertically and firmly on the stand.
  • The principal axis should be horizontal.
  • Use a stable platform to avoid vibrations.
  1. Distant Object:
  • Choose a truly distant object (at least 10 meters away, preferably farther).
  • The object should be well-illuminated for a clear image.
  • Do not use nearby objects as they are not at infinity.
  1. Screen Position:
  • The screen should be perpendicular to the principal axis.
  • Move the screen slowly and carefully to locate the sharp image.
  • Avoid parallax error while observing the image.
  1. Measurement:
  • Measure the distance carefully using a metre scale.
  • Place the metre scale parallel to the principal axis.
  • For concave mirror: Measure from the pole (P) to the screen.
  • For convex lens: Measure from the optical center (O) to the screen.
  • Take readings to the nearest millimeter (0.1 cm).
  1. Image Quality:
  • Ensure the image is the sharpest and smallest possible.
  • If the image is blurred, the screen is not at the correct position.
  • The image should be inverted and highly diminished.

Safety Precautions:

  1. Handling Mirrors and Lenses:
  • Handle with clean, dry hands.
  • Avoid touching the reflecting/refracting surfaces.
  • Do not drop or scratch the mirror/lens.
  1. Using Sunlight:
  • Never look directly at the sun through the lens or mirror.
  • Be careful when focusing sun rays as they can be intense enough to burn paper.
  • Do not leave the focused image on the screen for long to avoid burning.
  1. Eye Safety:
  • Do not place your eye in the path of reflected/refracted rays.
  • Wear safety goggles if required.

Accuracy:

  1. Repetition:
  • Repeat the experiment at least 3 times for accuracy.
  • Use different distant objects for each trial.
  • Calculate the mean value.
  1. Environmental Conditions:
    • Perform the experiment in good lighting conditions.
    • Avoid windy conditions that may shake the apparatus.
    • Ensure no obstructions between the distant object and mirror/lens.
  2. Parallax Error:
    • View the image and screen perpendicularly to avoid parallax error.
    • Mark the position carefully when the sharpest image is obtained.

SOURCES OF ERROR:

Possible Sources:

  1. The mirror/lens may not be perfectly vertical
  2. The principal axis may not be perfectly horizontal
  3. The distant object may not be far enough (not at infinity)
  4. The screen may not be perpendicular to the principal axis
  5. Parallax error in measuring distance
  6. Zero error in metre scale not accounted for
  7. The image may not be at the sharpest position
  8. Vibrations or movement of apparatus during measurement
  9. Poor lighting conditions
  10. Scratches or dirt on mirror/lens surface

VIVA QUESTIONS:

1. Q: What is the focal length of a mirror/lens?

  • A: The distance between the pole (for mirror) or optical center (for lens) and the principal focus.

2. Q: What is a concave mirror?

  • A: A spherical mirror whose reflecting surface is curved inward.

3. Q: What is a convex lens?

  • A: A lens that is thicker at the center and thinner at the edges, which converges light rays.

4. Q: Why do we use a distant object in this experiment?

  • A: A distant object is considered to be at infinity, so the rays coming from it are parallel. These parallel rays converge at the focus after reflection/refraction.

5. Q: What is the nature of the image formed by a concave mirror for a distant object?

  • A: Real, inverted, highly diminished (point-sized), formed at the focus.

6. Q: What is the nature of the image formed by a convex lens for a distant object?

  • A: Real, inverted, highly diminished (point-sized), formed at the focus.

7. Q: What is the relationship between radius of curvature and focal length of a concave mirror?

  • A: R = 2f (Radius of curvature is twice the focal length)

8. Q: Where should the screen be placed to get a sharp image?

  • A: At the principal focus of the mirror/lens.

9. Q: Why should the screen be perpendicular to the principal axis?

  • A: To get a clear and undistorted image on the screen.

10. Q: What happens if the object is not distant enough?

  • A: The rays will not be parallel, and the image will not form exactly at the focus, leading to error in focal length measurement.

11. Q: Can we use this method for a convex mirror?

  • A: No, because a convex mirror forms a virtual image that cannot be obtained on a screen.

12. Q: Can we use this method for a concave lens?

  • A: No, because a concave lens diverges rays and forms a virtual image that cannot be obtained on a screen.

13. Q: What is meant by ‘object at infinity’?

  • A: An object that is very far away (practically more than 50 times the focal length) such that the rays coming from it are parallel.

14. Q: Why is the image highly diminished?

  • A: Because the object is at infinity and the image is formed at the focus, which is very close to the mirror/lens compared to the object distance.

15. Q: What is the unit of focal length?

  • A: Centimeter (cm) or meter (m)

16. Q: What is the sign convention for focal length?

  • A:
    • Concave mirror: f is negative (−)
    • Convex lens: f is positive (+)

17. Q: What are some examples of distant objects?

  • A: Sun, distant tree, distant building, tower, mountain, etc.

18. Q: Why should we not look directly at the sun?

  • A: Direct sunlight can damage the retina and cause permanent eye damage or blindness.

19. Q: What is the principal focus?

  • A: The point on the principal axis where parallel rays converge (or appear to diverge from) after reflection/refraction.

20. Q: How many times should you repeat the experiment?

  • A: At least 3 times to minimize errors and obtain an accurate mean value.

DIAGRAMS:

Note: Draw neat, labeled ray diagrams showing:

  1. Concave mirror with parallel rays converging at focus
  2. Convex lens with parallel rays converging at focus
  3. Mark pole/optical center, focus, principal axis, and screen position

Signature of Teacher: ___

Date: ___

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