What is Concave Mirror and Convex Lens?
Concave Mirror
A concave mirror, which is also known as a converging mirror, consists of a reflecting surface that has been recessed inward (away from the incident light). The concave mirror can reflect the light inward to one focal point. They are used for focusing light. In a concave mirror, the image type depends on the distance between the object and the mirror.
Convex Lens
A convex lens is a type of lens that is thicker at the center and thinner at the edges. An optical lens is built using two spherical surfaces which if bent outwards will be considered as a convex lens. These types of lenses are used for converging a beam of light and focusing it to a point at the other side.
How to Find the Focal Length of a Concave Mirror - Class 12?
The following are the ways by which students can find the focal length of a concave mirror using a convex lens.
A concave mirror can be termed as a spherical mirror consisting of a reflecting surface that is curved inwards, and it also follows the laws of reflection of light.
The light rays that are coming from a distant object are considered to be parallel to each other.
If the image formed is real, inverted, and very small in size, then it is believed that the rays of light which are parallel to each other meet the point in the front of the mirror.
The image that is formed by the concave mirror is considered to be real and can be obtained on the screen.
‘f’ is used to express the difference between the principal axis P and the focus F of the concave mirror.
Materials Required to Find the Focal Length of Concave Mirror Using Convex Lens
The following are the list of materials that are required to find the focal length of a concave mirror using a convex lens.
A concave mirror
A measuring scale
A screen holder
A mirror holder
A mirror stand
Procedure to Find the Focal Length of Concave Mirror Using Convex Lens
The distance between the distinct objects selected should be more than 50ft.
The object and the concave mirror that is placed on the mirror stand should be facing each other.
The screen must be fixed in front of the reflecting surface of the mirror in order to obtain a clean and sharp image.
The distance between the screen and the concave mirror can be determined by using a meter scale. This distance is the same as the focal length of the given concave mirror.
This above process should be repeated three times to obtain the average of the focal length.
How to Find the Focal Length of a Convex Mirror Using a Convex Lens?
The following are the ways that should be followed to find the focal length of a convex mirror using a convex lens.
The convex mirror is thicker in the middle and is thinner at the edges. It is also known as a converging mirror.
The refracted rays that are coming from the parallel beam of light converge on the other side of the convex mirror.
The image that is obtained by the focus of the lens will be real, inverted, and very small in size.
‘F’ is known as the focal length, which is considered as the distance between the optical center of the lens and the principal focus.
There is a possibility of the image being formed on the screen if the image formed by the mirror is real.
Materials Required to Find the Focal Length of Convex Mirror Using a Convex Lens
The following are the list of materials that are used to find the focal length of a convex mirror using a convex lens.
A wooden bench
A convex lens
A lens holder
A screen fixed to a stand
A measuring scale
Procedure to Find the Focal Length of a Convex Mirror Using a Convex Lens
Try to arrange the lens and screen on the wooden bench without disturbing them.
The lens should be placed in a holder facing the distant object.
The holder should be placed with the screen on the bench.
The position of the screen should be such that the sharp image of the distant object is obtained easily without any difficulties.
The difference between the screen and the position of the lens is considered to be the focal length of the convex mirror.
After completing the above steps, shift the focus towards other distant objects in order to calculate the focal length of the convex mirror.
How is the Concave Mirror Different from a Convex Mirror?
The concave and convex mirrors are different from each other and therefore, give different results. If the outer surface of a spherical mirror is painted then it is known as a concave mirror whereas if the inner surface of a spherical mirror is painted, then it is known as a convex mirror. The concave mirror and convex mirror are also known as the converging mirrors and the diverging mirrors respectively. In the case of the concave mirror, only the inner surface is reflexive whereas, in the case of a convex mirror, only the outer surface is reflexive.
In the case of the concave mirror, the focal length is positive whereas the focal length is negative in the case of a convex mirror. Concave mirrors can form both real and virtual images and thus can be obtained on the screen whereas convex mirrors can form only virtual images which cannot be obtained on the screen. The center of curvature and the reflecting surface fall on the same side in a concave mirror whereas in the case of a convex mirror the center of curvature and the reflecting surface fall on the opposite sides. Vehicle headlights, shaving mirrors, and torches use concave mirrors whereas convex mirrors are used as rear view mirrors in bikes and cars. Magnified images of the object are obtained from a concave mirror whereas a convex mirror provides a wider view of the object.
Conclusion
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FAQs on Determination of Focal Length of Concave Mirror and Convex Lens
1. What is the correct experimental method to determine the focal length of a concave mirror according to CBSE 2025–26 guidelines?
The focal length of a concave mirror is determined by placing the mirror on a stand facing a well-illuminated distant object. A screen is positioned in front of the mirror to obtain a sharp, real, and inverted image. The distance between the mirror's pole and the screen, measured using a meter scale, gives the focal length. This measurement should be repeated at least three times with different objects, and the average value is taken as the focal length.
2. How can you find the focal length of a convex lens using the distant object method?
To determine the focal length of a convex lens, the lens is placed upright in a holder on a wooden bench, facing a distant, well-lit object. A screen is adjusted behind the lens until a sharp, real, and inverted image forms. The distance between the lens's optical center and the screen is measured—this is the focal length. Multiple trials with various distant objects give an average focal length.
3. What precautions should be taken during the focal length experiment of a concave mirror?
- Place the concave mirror near a window, facing a distant, bright object.
- Ensure the reflecting surface and object are facing each other directly.
- Keep the mirror, holder, and screen perpendicular or parallel to the meter scale as appropriate.
- Remove any obstacles to the light passing from the object to the mirror.
- Record measurements carefully and repeat the experiment for accuracy.
4. Why must the object be at a large distance when determining the focal length of a concave mirror or convex lens?
When the object is placed far away, the incident light rays can be considered parallel to the principal axis. This ensures the image forms at the focal point, allowing the measured distance from the mirror or lens to the image (screen) to accurately represent the focal length as defined in physics for spherical mirrors and lenses.
5. What are common sources of error in the determination of focal length in school laboratories?
- Improper alignment of the mirror/lens, screen, and measuring scale.
- Parallax error while measuring distances.
- Disturbance while adjusting the screen or lens during observation.
- Use of a poorly illuminated or moving distant object.
- Rough or uneven surface of the supporting bench causing instability.
6. How does the nature of the image formed differ between a concave mirror and a convex lens in these experiments?
A concave mirror forms a real, inverted, and diminished image on the screen when the object is distant. A convex lens also produces a real, inverted, and much smaller image at the focal plane for a distant object. Both allow direct measurement of focal length using the sharp image on a screen.
7. Compare the concept of focus and focal length for concave mirrors and convex lenses.
For a concave mirror, the focus is the point where parallel rays converge after reflection, and the focal length is the distance from the pole to the focus. For a convex lens, the focus is where parallel rays converge after refraction, and the focal length is the distance from the optical center to the principal focus on the opposite side. The methods of measurement and principles are analogous but involve different surfaces and refraction/reflection phenomena.
8. What are the theoretical principles behind measuring the focal length of concave mirrors and convex lenses in CBSE Physics labs?
- Both experiments use the fact that parallel rays from a distant object converge at the focus after reflection (mirror) or refraction (lens).
- The measured image distance from the optical element (mirror pole or lens center) to the screen is the focal length.
- Practically, as the object moves further away, the focus becomes more precise, validating the theoretical concept of the focal point.
9. If only a blurred image is obtained on the screen during the experiment, what should you check or adjust?
- Confirm the alignment of the mirror or lens and the screen is correct.
- Ensure that the object is sufficiently distant and well-lit.
- Check for stable platform placement and absence of vibrations.
- Gradually adjust the screen or lens position to find the sharpest image.
10. Why is it important to repeat the focal length measurement multiple times and take the average?
Repeating measurements accounts for minor experimental errors, helps compensate for slight misalignments or parallax, and yields a more accurate value for focal length as required by CBSE 2025–26 lab practices.
