# Concave and Convex Lenses: Image Formation

Spherical lenses are the lenses formed by bounding two spherical transparent surfaces together. In general there are two types of spherical lenses. So, lenses formed by binding two spherical surfaces bulging outward are known as convex lenses while the lenses formed by binding two spherical surfaces such that they are curved inward are known as concave lenses. This is one of the basic difference between concave and convex lenses.

Convex lenses are also known as converging lens since the rays converge after falling on the convex lens while the concave lenses are known as diverging lenses as the rays diverge after falling on the concave lens. ## Guidelines for rays falling on the concave and convex lenses

1. When a ray strikes concave or convex lenses obliquely at its pole, it continues to follow its path.
2. When a ray, parallel to principal axis strikes concave or convex lenses, the reflected ray passes through a focus on the principle axis.
3. When a ray, passing through focus strikes concave or convex lenses, the reflected ray will pass parallel to the principal axis.

## Image Formation by Concave and Convex Lenses:

### Convex Lenses

• When an object is placed at infinity, the real image is formed at the focus. The size of the image is much smaller as compared to that of the object. • When an object is placed behind the center of curvature, the real image is formed between the center of curvature and focus. The size of the image is same as compared to that of the object. • When an object is at the center of curvature, the real image is formed at the other center of curvature. The size of the image is same as compared to that of the object. • When an object is placed in between the center of curvature and focus, the real image is the formed behind the center of curvature. The size of the image is smaller as compared to that of the object. • When an object is placed at the focus, a real image is formed at infinity. The size of the image is much larger as compared to that of the object. • When an object is placed in between focus and pole, a virtual image is formed. The size of the image is larger as compared to that of the object. ### Concave Lenses

• When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is much smaller as compared to that of the object. • When an object is placed at a finite distance from the lens, a virtual image is formed between pole and focus of the convex lens. The size of the image is smaller as compared to that of the object. 