Edexcel GCSE Physics (1PH0) — Light and the electromagnetic spectrum
Why does a hot water bottle only warm your skin, while a microwave oven can heat food right through to the middle? The answer lies in how different waves interact with the body. Both of these waves are forms of non-ionising radiation, meaning they lack the energy to remove electrons from atoms. Because of this, they do not cause genetic mutations, and their hazards are purely thermal.
Microwaves penetrate the body's outer layers, reaching depths of up to 5 centimetres. They are absorbed by water molecules and fat molecules within the tissues, causing them to vibrate rapidly (resonance). This transfers energy to the thermal energy store of the tissues, resulting in the internal heating of body cells. This penetrating heat is dangerous as it can damage internal organs or boil internal fluids.
In contrast, infrared radiation does not penetrate deeply and is absorbed entirely by the surface cells of the skin. First, the radiation transfers energy directly to the thermal energy store of the skin tissue. Then, the kinetic energy and temperature of these outer cells rapidly increase. Finally, prolonged or intense exposure leads directly to painful skin burns.
The light that causes a painful sunburn is completely invisible to the human eye. Ultraviolet (UV) radiation carries higher energy than visible light and is readily absorbed by exposed skin and eyes.
Because it carries significant energy, UV can cause ionisation. First, the radiation is absorbed by surface cells or the delicate tissues of the eye (which can lead to cataracts). Then, it knocks electrons out of atoms, which leads to DNA damage within the cellular structures. Next, if the cell repairs this DNA incorrectly, it results in a permanent mutation. Finally, this mutation can trigger uncontrolled cell division, which manifests as skin cancer.
Alongside cancer, UV exposure causes premature skin ageing (a leathery texture). The body produces a natural pigment called melanin to absorb UV and protect deeper tissues, which is why skin tans when exposed to the sun. We can also use sunscreen to multiply our safe exposure time based on its SPF factor:
How long can a person safely stay in the sun if their skin normally burns in 12 minutes, assuming they generously apply an SPF 30 sunscreen?
Step 1: Identify the values.
Step 2: Substitute into the equation.
Step 3: Calculate the final answer.
X-rays and gamma rays are the highest-energy waves on the electromagnetic spectrum and are highly penetrating. Unlike UV waves that stop at the surface, these rays travel straight through soft tissue to reach internal organs and bones.
Because they are highly ionising, at high doses, they cause massive cell damage and immediate tissue death. At lower doses, the danger is genetic. First, the highly penetrating radiation reaches internal cells and causes ionisation of the atoms in their DNA strands. Then, the broken DNA strands cause a dangerous mutation. Finally, just like with UV rays, this mutation can lead to cancerous tumours deep within the body.
Students often incorrectly state that microwaves or infrared radiation cause cancer. Remember that these are non-ionising waves, so their dangers are purely thermal (internal heating or skin burns). Mark schemes will often explicitly tell examiners to ignore mentions of cancer for these waves.
In 6-mark questions describing the hazards of ionising radiation (UV, X-rays, or Gamma rays), use this specific sequence: Ionisation → DNA damage → Mutation → Uncontrolled cell division (Cancer).
For microwave hazards, Edexcel examiners look for the specific phrase 'internal heating of body cells' to award full marks.
When discussing UV, remember it is the only wave in the middle of the spectrum that causes both surface damage (like skin burns/sunburn) and ionising damage (cancer).
Non-ionising radiation
Radiation that lacks the energy to remove electrons from atoms; its hazards are thermal rather than mutagenic.
Internal heating
The process where radiation penetrates outer layers to transfer energy to internal molecules, causing a temperature rise from within body cells.
Skin burns
Damage to skin tissue caused by the transfer of thermal energy from infrared radiation to the surface cells.
Ionisation
The process of removing electrons from atoms or molecules to create charged ions, which can lead to DNA damage.
Mutation
A permanent change in a cell's genetic code (DNA) that can lead to abnormal cell function or uncontrolled division.
Skin cancer
A condition where skin cells divide uncontrollably, typically caused by DNA mutations from ionising ultraviolet radiation.
Melanin
A natural pigment in the skin that absorbs ultraviolet radiation to protect deeper tissues from ionising damage.
Put your knowledge into practice — try past paper questions for Physics
Non-ionising radiation
Radiation that lacks the energy to remove electrons from atoms; its hazards are thermal rather than mutagenic.
Internal heating
The process where radiation penetrates outer layers to transfer energy to internal molecules, causing a temperature rise from within body cells.
Skin burns
Damage to skin tissue caused by the transfer of thermal energy from infrared radiation to the surface cells.
Ionisation
The process of removing electrons from atoms or molecules to create charged ions, which can lead to DNA damage.
Mutation
A permanent change in a cell's genetic code (DNA) that can lead to abnormal cell function or uncontrolled division.
Skin cancer
A condition where skin cells divide uncontrollably, typically caused by DNA mutations from ionising ultraviolet radiation.
Melanin
A natural pigment in the skin that absorbs ultraviolet radiation to protect deeper tissues from ionising damage.
