  Searches for gravitational waves with km-scale laser interferometers often
involve the long-wavelength approximation to describe the detector response.
The prevailing assumption is that the corrections to the detector response due
to its finite size are small and the errors due to the long-wavelength
approximation are negligible. Recently, however, Baskaran and Grishchuk (2004
Class. Quantum Grav. 21 4041) found that in a simple Michelson interferometer
such errors can be as large as 10 percent. For more accurate analysis, these
authors proposed to use a linear-frequency correction to the long wavelength
approximation. In this paper we revisit these calculations. We show that the
linear-frequency correction is inadequate for certain locations in the sky and
therefore accurate analysis requires taking into account the exact formula,
commonly derived from the photon round-trip propagation time. Also, we extend
the calculations to include the effect of Fabry-Perot resonators in the
interferometer arms. Here we show that a simple approximation which combines
the long-wavelength Michelson response with the single-pole approximation to
the Fabry-Perot transfer function produces rather accurate results. In
particular, the difference between the exact and the approximate formulae is at
most 2-3 percent for those locations in the sky where the detector response is
greater than half of its maximum value. We analyse the impact of such errors on
detection sensitivity and parameter estimation in searches for periodic
gravitational waves emitted by a known pulsar, and in searches for an isotropic
stochastic gravitational-wave background. At frequencies up to 1 kHz, the
effect of such errors is at most 1-2 percent. For higher frequencies, or if
more accuracy is required, one should use the exact formula for the response.
