The human eye can respond to as little as 10^−18 j of light energy. for a wavelength at the peak of visual sensitivity, 550 nm, how many photons lead to an observable flash?

3 photons

Explanation:

The energy of a photon (E) can be calculated by the expression:

E = h*c/λ

Where h is the Planck constant (6.63x10⁻³⁴ J.s), λ is the wavelength (550 nm = 5.50x10⁻⁷m), and c is the speed of the light (3.00x10⁸ m/s).

E= (6.63x10⁻³⁴ * 3.00x10⁸)/(5.50x10⁻⁷)

E = 3.62x10⁻¹⁹ J

Which is the energy of one photon. The human eye can respond to 10⁻¹⁸ J, so:

1 photon 3.62x10⁻¹⁹ J

x               10⁻¹⁸ J,

By a simple direct three rule

x = 2.76 ≅ 3 photons.

force is based upon mass and acceleration.the statement is not always true.

it can be explained from newton's second law of motion.

as per newton's second law of motion the rate of change of momentum is directly proportional to the applied force and takes place along the direction of force.

mathematically it can be written as -

                                              ⇒      

here   k is the proportionality constant whose value is 1.

                                                            [a is the acceleration=dv/dt]

hence we see that force not only depends on mass and acceleration but also on velocity and rate of change of mass. force f will depend on mass and acceleration only when mass is constant.

hence the statement that force is based upon mass and acceleration is false.