# A photon of wavelength 100mm acts as a mass of? My text book gives the answer 2.21×10^-41. but a photon can't have mass because when an object travels with speed of light it's mass becomes infinite.

Aug 5, 2018

See below

#### Explanation:

$E = \frac{h c}{l} a m \mathrm{da}$

$E = \frac{\left(6.63 \cdot {10}^{- 34} J s\right) \left(3 \cdot {10}^{8} \frac{m}{s}\right)}{0.1 m} = 1.989 \cdot {10}^{- 24} J \mathmr{and} k g {m}^{2} / {s}^{2}$

$E = m {c}^{2}$

$\frac{E}{c} ^ 2 = m$

$m = \frac{1.989 \cdot {10}^{-} 24 J}{3 \cdot {10}^{8} \frac{m}{s}} ^ 2$

$m = 2.21 \cdot {10}^{- 41} k g$

And you're right about a photon being maseless, this is an incorrect application of $E = \Delta m {c}^{2}$

https://socratic.org/questions/can-you-thoroughly-describe-sigmund-freud-s-contributions-to-psychology?source=search

Aug 5, 2018

"a photon can't have mass because when an object travels with speed of light it's mass becomes infinite"

None of that makes any sense

EG:

• A photon is said to have zero rest-mass, as in, in its own reference frame it is massless.

So how do you reason that it can gain infinite mass? You are grossing up from zero!