At sunset, light travels through a longer path in the atmosphere. Shorter wavelengths (blue) scatter more (Rayleigh scattering), leaving longer wavelengths (red) to reach the observer.

Using v = u + at, where u = 0, v = 20 m/s, a = 2 m/s². 20 = 0 + 2t, so t = 10 seconds.

Gravitational force is always attractive, acts between any two masses, is independent of the medium, and is governed by Newton's law of universal gravitation.

When object distance (5 cm) < focal length (10 cm), the object is between the mirror and focus. A concave mirror forms a virtual, erect, and magnified image in this case.

For parallel resistances: 1/R = 1/R₁ + 1/R₂ = 1/10 + 1/20 = 3/20. Therefore R = 20/3 = 6.67 Ω.

Using v = fλ, where v = 330 m/s and f = 440 Hz. λ = v/f = 330/440 = 0.75 m.

Elastic potential energy E = (1/2)kx². 5 = (1/2)k(0.1)². 5 = 0.005k. k = 1000 N/m.

The photoelectric effect occurs when light of frequency greater than or equal to the threshold frequency strikes a metal surface, causing electron emission. It demonstrates the particle nature of light.

Kinetic energy KE = (1/2)mv². When velocity is doubled: KE' = (1/2)m(2v)² = (1/2)m(4v²) = 4 × (1/2)mv² = 4K.

The Second Law of Thermodynamics states that entropy of an isolated system increases or remains constant. Heat spontaneously flows from hot to cold, not vice versa, establishing the direction of processes.