Required practical activity 21: investigate how the amount of infrared…

The idea

Infrared is a region of the electromagnetic spectrum beyond red visible light. All objects emit infrared; the amount radiated or absorbed depends on surface colour and texture. In this practical, you compare surfaces like black matt, white matt and shiny silver. For absorption, place identical boiling tubes painted differently, filled with equal volumes of cold water, at equal distances from an infrared heater. Record the temperature rise over time. For emission (radiation), fill a Leslie cube with hot water and use an infrared detector at equal distances from each face to measure emitted radiation. Control variables include starting temperature, volume, distance, and time. Black matt surfaces are the best absorbers and emitters; shiny silver surfaces are the poorest. This explains why solar panels are black and survival blankets are shiny.

What earns the marks

• Identifies independent variable as surface nature (colour and texture) and dependent variable as amount of infrared absorbed or radiated, measured by temperature change or detector reading.
• Describes a valid method for absorption: identical containers with different surface coatings, equal volumes of cold water, placed at equal distances from an infrared heater, measuring temperature rise over time.
• Describes a valid method for emission: a Leslie cube or differently coated containers filled with hot water, using an infrared detector or thermometer to compare emitted radiation or cooling rates.
• States control variables: starting temperature, volume of water, distance from heater/detector, and time interval.
• Explains that black matt surfaces are the best absorbers and emitters of infrared, while shiny silver surfaces are the poorest.
• Uses data to compare rates of temperature change or detector readings and links the pattern to the nature of the surface.
• Evaluates the method, identifying sources of error such as heat loss to surroundings or inconsistent coating thickness, and suggests improvements.

Common mistakes and how to put them right

• Confusing the methods for absorption and emission.

  Put it right: Measuring the cooling of hot water tests emission; to test absorption, you must measure the heating of cold water using an external infrared source.

• Thinking shiny surfaces absorb infrared well because they reflect light.

  Put it right: Shiny silver surfaces reflect infrared too, so they absorb and emit less than black matt surfaces.

• Placing the infrared detector or heater at different distances from each surface.

  Put it right: Hold the detector or heater at the exact same distance, because infrared intensity decreases with distance.

• Recording only the final temperature rather than a series of readings.

  Put it right: Take regular readings so you can compare rates of cooling or heating, not just end points.

Exam tips

• When describing the method, clearly distinguish between testing absorption (heating cold water with an IR lamp) and testing emission (cooling hot water or using a Leslie cube).
• Name the independent, dependent and at least two control variables explicitly; this is often where marks are earned.
• Use comparative language such as 'black matt surface heated fastest' or 'shiny surface gave the lowest detector reading' rather than vague statements.