There are two sources of light, each emitting with a power of $100\ \text{W}$. One emits X-rays of wavelength $1\ \text{nm}$ and the other visible light at $500\ \text{nm}$. Find the ratio of number of photons of X-rays to the photons of visible light of the given wavelength.

There are two sources of light, each emitting with a power of 100W. One emits X-rays of wavelength 1nm and the

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100\ \text{W}

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Concept understanding — Photon Energy

Photon Energy: The Energy Carried by Light

Light does not carry its energy in a continuous stream. It comes in discrete packets — like individual drops instead of a steady hose. Each packet is called a photon: the smallest possible unit of light of a given frequency. You cannot have half a photon; it is all or nothing.

The Intuition: Colour Determines Energy

Red light and blue light are both light, but they behave differently — blue light causes sunburn and drives chemical reactions more readily than red. The reason is that the colour (frequency) of light directly fixes how much energy each photon carries, and blue photons carry more energy than red photons.

The Planck–Einstein Relation

The energy $E$ of a single photon is directly proportional to its frequency $f$ , and inversely proportional to its wavelength $\lambda$ :

E = h f = \frac{h c}{\lambda}

Where:

$E$ = energy of one photon (joules, J)
$f$ = frequency (hertz, Hz)
$\lambda$ = wavelength (metres, m)
$c$ = speed of light in vacuum ( $3.00 \times 10^8$ m/s)
$h$ = Planck's constant ( $6.626 \times 10^{-34}$ J·s)

Important

Planck's constant $h$ is extremely tiny, so an individual photon carries a very small amount of energy — which is why we never feel single photons striking our skin.

What This Tells You

Higher frequency = higher energy. Gamma rays have extremely energetic photons; radio waves have very low-energy photons.
Shorter wavelength = higher energy. Ultraviolet photons are more energetic than visible-light photons; infrared photons are less.
Energy is quantised. Light energy comes in fixed packets — you can have 1, 2, or 1000 photons, but never 0.5. This was Max Planck's revolutionary idea of 1900.

This is why UV light causes sunburn (UV photons carry enough energy to damage DNA, while visible photons do not), why X-ray photons are energetic enough to pass through soft tissue but get absorbed by bone, and why a photon needs enough energy to actually break a bond or drive a photochemical/biological reaction, rather than merely wavelength-dependent intensity.

A Concrete Example

Note

Question: Which has more energy — a photon of red light ( $\lambda = 700$ nm) or a photon of blue light ( $\lambda = 450$ nm)?

Since $E = \dfrac{hc}{\lambda}$ , energy is inversely proportional to wavelength, so the shorter-wavelength blue photon carries more energy:

Red: $E_{\text{red}} = \dfrac{(6.626 \times 10^{-34})(3.00 \times 10^8)}{700 \times 10^{-9}} \approx 2.84 \times 10^{-19}$ J …

Why this formula?

Why Photon Energy is $E = hf$ — The Reasoning Behind the Formula

The formula $E = hf$ is not something you memorise and plug numbers into. It comes from a deep shift in how physicists understood light.

The problem that forced the formula

By the late 1800s, classical physics said light was a wave — continuous, carrying energy in proportion to its intensity (brightness). But blackbody radiation and the photoelectric effect showed something bizarre: when you shine light on a metal, electrons are ejected only if the light's frequency is above a certain threshold, no matter how bright the light is. Below that frequency, even the brightest lamp won't kick out a single electron. Classically this made no sense — a wave's energy depends on amplitude, not frequency.

Einstein's radical idea (1905)

Einstein proposed that light comes in discrete packets — photons — each carrying a fixed energy that depends only on its frequency, with Planck's constant $h$ as the proportionality:

A single photon's energy is $E = hf$ , where $f$ is the frequency of the light.

This directly explains the photoelectric effect: an electron needs a minimum energy (the work function $\phi$ ) to escape. If a photon's energy $hf < \phi$ , no electron is ejected — regardless of how many photons you send, because each photon interacts with one electron individually.

Important

$E = hf$ is not derived from deeper principles — it is a postulate justified by the experiments it explains, and it unified optics with quantum mechanics.

The equivalent forms

Since $c = f\lambda$ for light in vacuum, the same idea in terms of wavelength is:

E = \frac{hc}{\lambda}

Use this when you're given $\lambda$ instead of $f$ . A photon also carries momentum (despite having no mass): …

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The ratio of the number of photons emitted per second by the X-ray source to that by the visible light source is $1:500$ . This follows directly from the fact that for equal power, the photon emission rate is inversely proportional to the photon energy, which itself is inversely proportional to the wavelength.

Why photon energy is the key

Both sources deliver the same power — $100\ \text{W}$ — meaning each transfers $100$ joules of energy per second. But a single X-ray photon carries far more energy than a single visible-light photon because energy per photon is inversely proportional to wavelength. So to deliver the same total energy per second, the visible source must emit many more photons. The ratio of photon counts is simply the inverse ratio of their energies.

E = \frac{hc}{\lambda}

Step-by-step solution

Write the relation between power, photon energy, and number of photons per second. If a source emits $n$ photons per second, each of energy $E$ , the total power is

P = n \cdot E = n \cdot \frac{hc}{\lambda}.

Since $P$ is the same for both sources, we have

n_{\text{X}} \cdot \frac{hc}{\lambda_{\text{X}}} = n_{\text{V}} \cdot \frac{hc}{\lambda_{\text{V}}}.

Cancel the common factor $hc$ . This gives

\frac{n_{\text{X}}}{\lambda_{\text{X}}} = \frac{n_{\text{V}}}{\lambda_{\text{V}}}.

Rearrange to find the ratio of photon numbers. $\frac{n_{\text{X}}}{n_{\text{V}}} = \frac{\lambda_{\text{X}}}{\lambda_{\text{V}}}.$ …

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Q.There are two sources of light, each emitting with a power of $100\ \text{W}$ . One emits X-rays of wavelength $1\ \text{nm}$ and the other visible light at $500\ \text{nm}$ . Find the ratio of number of photons of X-rays to the photons of visible light of the given wavelength.

Concept understanding — Photon Energy

Photon Energy: The Energy Carried by Light

The Intuition: Colour Determines Energy

The Planck–Einstein Relation

What This Tells You

A Concrete Example

Why this formula?

Why Photon Energy is $E = hf$ — The Reasoning Behind the Formula

The problem that forced the formula

Einstein's radical idea (1905)

The equivalent forms

Why photon energy is the key

Step-by-step solution

Method: Photon-Count Ratio at Equal Power

Steps

Why photon energy is the key

Step-by-step solution

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