Objects reflect the color they appear to be. A green leaf reflects green light and absorbs other colors.

In visible light, red color has the maximum wavelength ($\lambda$) and minimum frequency ($u$). Violet color has the minimum wavelength and maximum frequency. The speed of light in a medium depends on the color and refractive index.

The splitting of white light into seven colors is called 'Dispersion'.

Explanation - Atmospheric refraction is the deviation of light or other electromagnetic waves from a straight line as it passes through the atmosphere due to variations in air density as a function of height. This refraction is caused by the velocity of light in air with increased density. Near the ground, atmospheric refraction creates a mirage. This is the reason why stars appear to twinkle to us and the moon appears larger near the horizon.

The pinhole narrows the light rays and increases the 'depth of field', reducing blurring. Practically, it produces an effect similar to a convex lens which helps in clear focus on the retina.

Explanation - The refractive index of different layers of air in the atmosphere is different. Refraction of light occurs, which causes stars to twinkle at night. As light moves from one medium to another (denser to rarer), its speed changes. Layers of air with different densities are found in the atmosphere. When light travels from a star, it undergoes continuous refraction in different layers. Since the layers of air are not stable, it is perceived that the star is in different positions at different intervals, i.e., the star appears to twinkle.

Parabolic concave mirrors used in vehicle headlights are utilized to collect energy from a distant source and bring it to a common focal point. Parabolic mirrors used in headlights project the energy coming from a source outward as a parallel beam of light.

In light, the three primary colors are Red, Green, and Blue (RGB). Color televisions use a mixture of these colors to create all other colors.

When the object is between the focus point (F) and the optical center (O), the image is formed on the same side of the lens, which is virtual, erect, and magnified.

Red color has the least deviation. Among the given options, Orange color is closest to red, so it will have the least deviation.

If three thin lenses are placed in contact with each other, the effective power (P) of the lens system is $P = \frac{1}{f_1} + \frac{1}{f_2} + \frac{1}{f_3} \Rightarrow \frac{1}{20} + \frac{1}{50} - \frac{1}{25} = \frac{100}{20} + \frac{100}{50} - \frac{100}{25} (\text{converting to meters}) \Rightarrow P = 5 + 2 - 4 = +3D \text{ (Diopter)}$

The process where the left part appears as right and right part as left in an image formed by a mirror is called lateral inversion.

For a concave lens, focal length is negative, so $f = -50$ cm = $-0.5$ meters. Power $P = 1/f = 1/-0.5 = -2D$.

When the object is placed at the center of curvature (C) of a concave mirror, its image is also formed at the center of curvature (C). This image is real, inverted, and equal in size to the object.

According to geometric calculation, when two mirrors are perpendicular to each other, the angle between the reflected ray and the incident ray is $120^{\circ}$.

The image formed by a concave lens is always virtual, which cannot be obtained on a screen.

The blue color of the sky is due to the scattering of light. Particles in the atmosphere (like gas molecules, dust) scatter sunlight in different directions, and since blue light has a shorter wavelength, it scatters more.

Primary colors are those colors which cannot be created by mixing other colors. Red, green, and blue are called primary colors.

The image formed by a convex mirror is always virtual and erect.

A concave mirror is the only mirror that can form an inverted image larger than the object (when the object is between the focus and the center of curvature).