A convex lens is thicker at the centre than at the edges.
Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge). A convex lens is a converging lens. When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus. The distance between the principal focus and the centre of the lens is called the focal length. |
Use of Convex Lenses – The Camera
A camera consists of three main parts.
- The body which is light tight and contains all the mechanical parts.
- The lens which is a convex (converging) lens).
- The film or a charged couple device in the case of a digital camera.
The rays of light from the person are converged by the convex lens forming an image on the film or charged couple device in the case of a digital camera. The angle at which the light enters the lens depends on the distance of the object from the lens. If the object is close to the lens the light rays enter at a sharper angled. This results in the rays converging away from the lens. As the lens can only bend the light to a certain agree the image needs to be focussed in order to form on the film. This is achieved by moving the lens away from the film. Similarly, if the object is away from the lens the rays enter at a wider angle. This results in the rays being refracted at a sharper angle and the image forming closer to the lens. In this case the lens needs to be positioned closer to the film to get a focused image. Thus the real image of a closer object forms further away from the lens than the real image of a distant object and the action of focusing is the moving of the lens to get the real image to fall on the film. The image formed is said to be real because the rays of lighted from the object pass through the film and inverted (upside down). |
The Magnifying Glass
A magnifying glass is a convex lens which produces a magnified (larger) image of an object.
A magnifying glass produces an upright, magnified virtual image. The virtual image produced is on the same side of the lens as the object. For a magnified image to be observed the distance between the object and the lens must be shorter than the focal length of the lens.
A magnifying glass is a convex lens which produces a magnified image of an object. |
For a magnified image to be observed the distance between the object and the lens has to be shorter than the focal length of the lens. The image formed is upright, magnified and virtual. |
Magnification
The magnification of a lens can be calculated using the following formula;
As this is a ratio between heights it has no units. A magnification of 2 means the image is twice the size of the object and a magnification of 1 indicates an image size being the same as the object size.
Concave Lens
A concave lens is thinner at the centre than at the edges.
Concave lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A concave lens is a diverging lens. When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus. The distance between the principal focus and the centre of the lens is called the focal length. The image formed is virtual and diminished (smaller) |
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Answer
Hint:Determine the image distance using the formula for magnification of the lens. Using the lens formula, determine the focal length of the convex lens. It should be a positive. The radius of curvature is twice the focal length of the lens.
Formula used:
Magnification, \[m = \dfrac{{{h_i}}}{{{h_o}}}\] Here, \[{h_i}\] is the height of the image and \[{h_o}\] is the height of the object.Lens formula, \[\dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u}\]Here, f is the focal length, u is the object distance and v is the image distance.Complete step by step answer:
Therefore, the radius of curvature of the convex lens is 60 cm.
Note: Students must be able to recognize the nature of the lens using the sign of focal length. The convex lens has a positive focal length while the concave lens has a negative focal length. Also, the object distance is always negative for both convex lens and concave lens while the image distance is positive for real image formed by convex lens.
Spherical lenses are lenses formed by connecting two spherical transparent surfaces. 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 lenses since the rays converge after falling on the convex lens. The concave lenses are known as diverging lenses, as the rays diverge after falling on the concave lens. In this article, we will learn about image formation by concave and convex lenses.
- When a ray strikes concave or convex lenses obliquely at its optical centre, it continues to follow its path.
- When a ray, parallel to the principal axis strikes concave or convex lenses, the reflected ray passes through a focus on the principal axis.
- When a ray, passing through focus strikes concave or convex lenses, the reflected ray will pass parallel to the principal axis.
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- When an object is placed at infinity, the real image is formed at the focus. The size of the image is highly diminished and point size.
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- When an object is placed beyond the centre of curvature, the real image is formed between the centre of curvature and focus. The image size will not be the same as the object. It will be diminished in size..
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- When an object is at the centre of curvature, the real image is formed at the other centre of curvature. The size of the image is the same as compared to that of the object.
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- When an object is placed in between the centre of curvature and focus, the real image is formed behind the centre of curvature. The size of the image is larger than that of the object.
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- When an object is placed at the focus, a real image is formed at infinity. The size of the image is much larger than that of the object.
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- When an object is placed in between focus and optical centre, a virtual image is formed. The size of the image is larger than that of the object.
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- When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is highly diminished and point size.
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- When an object is placed at a finite distance from the lens, a virtual image is formed between the optical centre and the focus of the convex lens. The size of the image is smaller than that of the object.
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