Class 10 Science Chapter 2: Key Points to Remember on Optics

Optics: Key Points to Remember

Chapter 2: Optics Summary

• Light is a form of energy which travels along a straight line.
• The deviation in the path of light ray is called refraction.
• The ratio of speed of light in vacuum to the speed of light in a medium is defined as refractive index ‘µ’ of that medium.
• Lens formula:
• Magnification (m) is given by \( m = \frac{h'}{h} = \frac{v}{u} \).
• Power of lens, \( P = \frac{1}{f} \).
• The ability of the eye to focus nearby as well as the distant objects is called power of accommodation of the eye.
• A microscope is an optical instrument which helps us to see the objects which are very small in dimension.
• Telescope is an optical instrument used to see the distant objects clearly.

Understanding Magnification of a Lens: Formula and Concepts for Class 10 Optics

Magnification of a Lens

MAGNIFICATION OF A LENS

Like spherical mirrors, we have magnification for spherical lenses. Spherical lenses produce magnification and it is defined as the ratio of the height of the image to the height of an object. Magnification is denoted by the letter ‘m’. If height of the object is h and height of the image is h´, the magnification produced by lens is,

Also it is related to the distance of the object (u) and the distance of the image (v) as follows:

If the magnification is greater than 1, then we get an enlarged image. On the other hand, if the magnification is less than 1, then we get a diminished image.

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Context: 10th Science : Chapter 2 : Optics : Magnification of a Lens

Lens Formula Explained: A Key Concept in 10th Science Optics

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Lens Formula

10th Science : Chapter 2 : Optics : Lens Formula

Defining the Lens Formula

LENS FORMULA

Like spherical mirrors, we have lens formula for spherical lenses. The lens formula gives the relationship among distance of the object (u), distance of the image (v) and the focal length (f) of the lens. It is expressed as

$$ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} $$

Application and Key Considerations

It is applicable to both convex and concave lenses. We need to give an at most care while solving numerical problems related to lenses in taking proper signs of different quantities.

Understanding Refraction Through a Concave Lens: Image Formation Explained

Refraction Through a Concave Lens

REFRACTION THROUGH A CONCAVE LENS

Let us discuss the formation of images by a concave lens when the object is placed at two possible positions.

Object at Infinity

When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is much smaller than that of the object (Figure 2.12).

Figure 2.12 Concave lens-Object at infinity

Object anywhere on the principal axis at a finite distance

When an object is placed at a finite distance from the lens, a virtual image is formed between optical center and focus of the concave lens. The size of the image is smaller than that of the object (Figure 2.13).

Figure 2.13 Concave lens-Object at a finite distance

But, as the distance between the object and the lens is decreased, the distance between the image and the lens also keeps decreasing. Further, the size of the image formed increases as the distance between the object and the lens is decreased. This is shown in (figure 2.14).

Figure 2.14 Concave lens- Variation in position and size of image with object distance