transmittance

Meaning: Transmittance is the intensity of light leaving a sample divided by the intensity that entered it. When the frequency of infrared radiation matches the vibration of a bond, that energy is absorbed and less light gets through, so transmittance drops. More absorption means less transmittance.

Equation: T = I/I₀, and A = −log₁₀T, where I is the transmitted intensity (W/m²), I₀ is the incident intensity (W/m²), T is transmittance (dimensionless, 0 to 1 or 0 to 100%), and A is absorbance (dimensionless). For example, T = 10% gives A = 1.

On the MCAT: IR spectra are drawn with % transmittance on the y-axis and wavenumber (cm⁻¹) on the x-axis, which runs from high to low left to right. Each absorption appears as a downward peak. You match peaks to functional groups, such as a strong, sharp C=O dip near 1700 cm⁻¹ or a broad O–H dip around 3300 cm⁻¹. A flat baseline near 100% means the sample absorbs little there. In UV-vis questions, Beer-Lambert (A = εlc) predicts that a higher concentration or longer path length lowers transmittance.

Common confusion: Students read the downward peaks as "low signal" or as absence of a bond. A deep dip means strong absorption, so the bond is present. Also, transmittance falls exponentially with concentration, while absorbance rises linearly, so doubling the concentration does not halve T.