Explanation

According to Einstein's photoelectric equation, the kinetic energy (K) of an ejected electron depends on the frequency (f) of the incident light:

K = hf - φ

where h is Planck's constant and φ is the work function of the metal.

Explanation

A convex lens can focus sunlight at one point by converging the light rays.

That is why it is often used in magnifying glasses or for starting fires using sunlight.

Explanation

Radio waves frequency is measured in Hertz.

Hertz (Hz) quantifies the number of cycles per second of a radio wave.

This determining its frequency and position within the electromagnetic spectrum.

Explanation

Ultrasound is sound with frequencies greater than 20 kilohertz.

This frequency is the approximate upper audible limit of human hearing in healthy young adults

while 

The first article on the history of ultrasound was written in 1948

Explanation

Angular acceleration is produced by torque.

Torque is the rotational equivalent of force.

And just as force causes linear acceleration, torque causes angular acceleration. 

Explanation

The slope of a velocity-time graph gives the rate of change of velocity, which is acceleration.

At any instant, this slope shows the instantaneous acceleration.

Explanation

Reverberation time is mainly influenced by:

• The volume of the room

• The absorptive properties of surfaces

• The number of sound-absorbing objects or people

While absorption can vary slightly with frequency, the frequency itself is not a primary controlling factor of reverberation time.

Explanation

Helium hydride (HeH+) was the first molecule discovered in space, identified in 2019 by astronomers.

It was found in a planetary nebula, marking a significant discovery in molecular chemistry in space.

Explanation

Mercury remains liquid at a wide range of temperatures, has uniform expansion.

It is easily visible due to its opacity.

It does not stick to glass, allowing accurate and consistent readings in thermometers.

Explanation

Let's denote the weight of the man in the stationary lift as W and the weight in the accelerating lift as W'.

We know that W' / W = 2/3.

When the lift is accelerating downward with acceleration 'a', the apparent weight (W') is given by:

W' = W - ma

where m is the mass of the man.

Since W' / W = 2/3, we can write:

(W - ma) / W = 2/3

Substitute W = mg (where g is the acceleration due to gravity):

(mg - ma) / mg = 2/3

Simplify:

g - a = (2/3)g

a = g - (2/3)g

a = (1/3)g

a = g/3