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Class 10 Light Formulas

This page provides a concise formula compilation on light reflection and refraction from Class 10 Science Chapter 9. A downloadable PDF formula sheet includes all essential formulas and key points. It facilitates efficient revision and subject mastery, serving as an invaluable study resource for board exams.

Revision Notes: Light – Reflection and Refraction

Reflection of Light

Laws of Reflection:

1. The angle of incidence ($i$) is equal to the angle of reflection ($r$).
2. The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.

Mirror Formula:
$$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$
Where:

• $f$ = focal length
• $v$ = image distance
• $u$ = object distance

Magnification (Mirrors):
$$m = \frac{h’}{h} = -\frac{v}{u}$$
Where:

• $m$ = magnification
• $h’$ = image height
• $h$ = object height

Sign Conventions for Mirrors:

• Object distance ($u$): Always negative
• Image distance ($v$): Positive for virtual images, negative for real images
• Focal length ($f$): Positive for convex mirrors, negative for concave mirrors
• Image height ($h’$): Positive for erect images, negative for inverted images

Refraction of Light

Laws of Refraction (Snell’s Law):
$$\frac{\sin i}{\sin r} = n$$
Where:

• $i$ = angle of incidence
• $r$ = angle of refraction
• $n$ = refractive index

Refractive Index:
$$n = \frac{c}{v}$$
Where:

• $c$ = speed of light in vacuum
• $v$ = speed of light in the medium

Relative Refractive Index:
$$n_{1,2} = \frac{V_2}{V_1}$$
Where $V_1$ and $V_2$ are the speeds of light in media 1 and 2 respectively.

Absolute Refractive Index:
$$n_m = \frac{c}{V}$$

Lenses

Lens Formula:
$$\frac{1}{f} = \frac{1}{v} – \frac{1}{u}$$
Where:

• $f$ = focal length
• $v$ = image distance
• $u$ = object distance

Magnification (Lenses):
$$m = \frac{h’}{h} = \frac{v}{u}$$

Power of a Lens:
$$P = \frac{1}{f}$$
Where $P$ is in diopters (D) and $f$ is in meters.

Net Power of Combined Lenses:
$$P_{\text{net}} = P_1 + P_2 + P_3$$
Where $P_1$, $P_2$, and $P_3$ are the powers of individual lenses.

Sign Conventions for Lenses:

• Object distance ($u$): Always negative
• Image distance ($v$): Positive for real images, negative for virtual images
• Focal length ($f$): Positive for convex lenses, negative for concave lenses
• Image height ($h’$): Positive for erect images, negative for inverted images

Magnification Sign Conventions:

• $m = -ve$ -> Inverted
• $m = +ve$ -> Erect
• $m < 1$ -> Diminished
• $m = 1$ -> Same size
• $m > 1$ -> Enlarged

All formulas and concepts in tabular form

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Tips for Exam Preparation:

1. Practice ray diagrams for both mirrors and lenses to understand image formation.
2. Memorize the sign conventions and apply them consistently.
3. Work through numerical problems using the mirror and lens formulas.
4. Understand the conceptual difference between real and virtual images.
5. Review the derivations of key formulas and practice applying them in different contexts.

These notes cover the essential formulas, sign conventions, and principles of reflection and refraction of light, which are critical for Class 10 exams. Adding diagrams at the appropriate places will help in visualizing the concepts better.

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