Image Formation by Spherical Lenses
Spherical lenses are used to form images of objects placed in front of them. The type of image formed by the lens depends on whether the lens is convex or concave and the position of the object relative to the focal point and the optical center of the lens.
Convex Lens Image Formation
A convex lens can form real or virtual images depending on the position of the object. Below are the cases for image formation by a convex lens:
- Object at infinity: A real, inverted, and highly diminished image is formed at the focus.
- Object beyond 2F (twice the focal length): A real, inverted, and diminished image is formed between F and 2F.
- Object at 2F: A real, inverted, and same-size image is formed at 2F.
- Object between F and 2F: A real, inverted, and enlarged image is formed beyond 2F.
- Object at F: No image is formed, as rays become parallel after refraction.
- Object between F and O (optical center): A virtual, upright, and enlarged image is formed on the same side as the object.
Concave Lens Image Formation
A concave lens always forms virtual, upright, and diminished images regardless of the object’s position. The image appears between the optical center and the focus on the same side as the object.
Lens Formula
Where:
- f = Focal length of the lens
- v = Image distance
- u = Object distance
Magnification Formula
Where:
- m = Magnification
- v = Image distance
- u = Object distance
- hi = Height of image
- ho = Height of object
Frequently Asked Questions (FAQs)
1. What type of image is formed by a convex lens when the object is placed at infinity?
A convex lens forms a real, inverted, and highly diminished image at its focus when the object is placed at infinity.
2. What is the focal length of a lens?
The focal length of a lens is the distance from the optical center of the lens to its focal point, where parallel rays of light converge or appear to diverge.
3. Can a concave lens form a real image?
No, a concave lens always forms virtual, upright, and diminished images.
4. What is the lens formula and what does it signify?
The lens formula is 1/f = 1/v – 1/u, and it relates the focal length of the lens to the image distance and object distance.
5. What is magnification in lenses?
Magnification is the ratio of the height of the image to the height of the object and is given by the formula m = v/u = hi/ho.
6. What type of image is formed by a convex lens when the object is placed between F and O?
A virtual, upright, and enlarged image is formed on the same side as the object.
7. What are the applications of convex lenses?
Convex lenses are used in magnifying glasses, cameras, microscopes, and corrective lenses for farsightedness.
8. Why can’t concave lenses form a real image?
Concave lenses diverge light rays, making them appear to come from a point behind the lens, which results in virtual images only.
9. What is the nature of an image formed by a convex lens when the object is placed at 2F?
A real, inverted, and same-size image is formed at 2F on the other side of the lens.
10. How is the focal length of a convex lens determined?
The focal length of a convex lens is determined using the lens formula and can be measured by placing an object at various distances from the lens and calculating the image distance.
